Previously ReturnPointer was just the first slot in the locals array,
which had type `Vec<Pointer>`. But after my recent refactoring, locals
is `Vec<Value>` and it became increasingly hacky to pull a pointer out
of the first slot to be the value. Besides, that hack wouldn't allow
ReturnPointer to ever be an `Lvalue::Local`, referring directly to a
local on a higher stack frame.
Now ReturnPointer has no presence in the locals array, instead being
upgraded to its own field on `Frame`.
This introduces a couple of new hacks, detailed by some of my FIXME
comments, so that I could get the tests passing again and commit. More
commits coming soon should clean up these hacks without much trouble,
and overall I feel that the code is converging on a cleaner, more
efficient design.
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.
