Turning locals into `Vec<Value>` will allow writing `PrimVal` results
directly into the locals array without creating `memory::Allocation`s
for every local.

This will entail passing around a generalized kind of `Lvalue` instead
of `Pointer`s for the destinations of operations. Replacing `Pointer`
with `Lvalue` is mostly done with this commit, but expanding `Lvalue`
will come later.

This commit turns every local from `Pointer` into `Value::ByRef(ptr)`.
Locals which are `Value::ByVal(prim_val)` will come in a later commit.

This reduces the number of allocations Miri makes drastically. The
`const_to_ptr` function was a lame hack that allocated for every since simple
constant, and all of those are avoided now, except for one extra allocation each
for string and bytestring literals which will be fixed in a followup commit.

There are a number of hacks such as `eval_operand_to_ptr` left over from this
commit, which will also be fixed in followup commits.

previously we moved to the target block *before* calling a function, so when inspecting
the stack, it appeared as if we were in the first statement of the next block.

This might create confusion, because attempting to execute a statement can cause
arbitrary stackframes to be added for the constants/statics/promoteds required by that
statement. Before this commit, the first statement of the last added stackframe was
executed immediately. Thus there was no way to inspect the state before that first
statement.