* Move some code outside dbms/src folder
* Fix paths

Instead, these methods return a pointer to the required data as they are
stored inside the hash table. The caller uses overloaded functions to
get the key and "mapped" values from this pointer. Such an interface
avoids the need for constructing iterator-like wrapper objects, which is
especially important for compound hash tables such as the future
StringHashMap.

Some aggregation methods initially emplace a temporary StringRef key
into a hash table. Then, if the key was not seen before, they make a
persistent copy of the key and update the hash table with it. This
approach is not suitable for compound hash tables, because the logic of
when the persistent key is needed is more complex, and is contained
within the hash table itself.

In this commit, we switch to managing key memory with callbacks passed
to the hash table, that allow it to request a persistent copy of the key
if it is needed. This should be more appropriate for compound hash
tables.

This commit prepares for StringHashMap PR #5417.

This is the first step of allowing heterogeneous cells in hash tables.

performance test results are

```

1. HashMap<UInt16, UInt8, TrivialHash, HashTableFixedGrower<16>>;
2. NewLookupMap<UInt16, UInt8>

ResolutionWidth 30000 1 .................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................223550276.46
ResolutionWidth 30000 2 .................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................248772721.24
Best: 2 - 24877272124

ResolutionWidth 100000 1 ..........................................................................................................................................................................................................................................................238498413.99
ResolutionWidth 100000 2 ..........................................................................................................................................................................................................................................................261808889.98
Best: 2 - 26180888998

ResolutionWidth 300000 1 ...................................................................................239307348.81
ResolutionWidth 300000 2 ...................................................................................257592761.30
Best: 2 - 25759276130

ResolutionWidth 1000000 1 .........................240144759.26
ResolutionWidth 1000000 2 .........................257093531.91
Best: 2 - 25709353191

ResolutionWidth 5000000 1 .....241573260.35
ResolutionWidth 5000000 2 .....259314162.79
Best: 2 - 25931416279

ResolutionDepth 30000 1 .................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................217108119.84
ResolutionDepth 30000 2 .................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................249459504.41
Best: 2 - 24945950441

ResolutionDepth 100000 1 ..........................................................................................................................................................................................................................................................229065162.17
ResolutionDepth 100000 2 ..........................................................................................................................................................................................................................................................253769105.64
Best: 2 - 25376910564

ResolutionDepth 300000 1 ...................................................................................233079225.18
ResolutionDepth 300000 2 ...................................................................................256316273.78
Best: 2 - 25631627378

ResolutionDepth 1000000 1 .........................234184633.51
ResolutionDepth 1000000 2 .........................261100491.57
Best: 2 - 26110049157

ResolutionDepth 5000000 1 .....233118795.66
ResolutionDepth 5000000 2 .....252436160.41
Best: 2 - 25243616041

```