* Fix nested set operations with leftmost nulls

Fixes #2366

Closes #1970
Closes #873
Closes #473
Closes #1974
Closes #1983

EFCore -> 6.0.0-preview.5.21230.1 (67a5d1697131086b9984c457cec944e8f957f2d8)
MicrosoftExtensions -> 6.0.0-preview.5.21226.1
Npgsql -> 6.0.0-ci.20210502T183829

Commit ca2a793016c6980c943325c214f42602910d9991

We now append NULLS LAST for descending sorts, rather than just NULLS
FIRST always.

Related to #50

Some work on type mapping to make everything work.
Every mapping now has to be one-to-one: a mapping returned for a CLR
type must contain that same type; previously when looking up by
DateTimeOffset we would return a timestamptz mapping which specified
CLR DateTime.

We now have separate mappings for DateTimeOffset and DateTime
(as well as for byte and short), and can handle multiple mappings
given a store type.

Closes #339

The functional tests were written for SQL Server and assume that
ORDER BY sorts nulls first, but PostgreSQL sorts nulls last by
default.

Add a context option that makes Npgsql generate NULLS FIRST after
ORDER BY. Since the full feature would also require creating
indexes with nulls first, this is (for now) an internal-only
feature.

Closes #50

Incomplete

Following preview3

All tests pass (apart from skipped)

All tests pass except for some EFCore-related issues.
Includes many various fixes.

Note that no tests have been executed and the provider is probably still
broken in some ways (this is just a first step).

Fixes #17

From commit bb43e2282fb13b0deb931d199c95d07ab84169a0.
Project does not compile.