Remove some allocations in CodeMemory (#3253)

* Remove some allocations in `CodeMemory`

This commit removes the `FinishedFunctions` type as well as allocations
associated with trampolines when allocating inside of a `CodeMemory`.
The main goal of this commit is to improve the time spent in
`CodeMemory` where currently today a good portion of time is spent
simply parsing symbol names and trying to extract function indices from
them. Instead this commit implements a new strategy (different from #3236)
where compilation records offset/length information for all
functions/trampolines so this doesn't need to be re-learned from the
object file later.

A consequence of this commit is that this offset information will be
decoded/encoded through `bincode` unconditionally, but we can also
optimize that later if necessary as well.

Internally this involved quite a bit of refactoring since the previous
map for `FinishedFunctions` was relatively heavily relied upon.

* comments

crates/environ/src/lib.rs

@@ -43,6 +43,7 @@ pub use crate::stack_map::StackMap;
 pub use crate::trap_encoding::*;
 pub use crate::tunables::Tunables;
 pub use crate::vmoffsets::*;
+pub use object;
 
 // Reexport all of these type-level since they're quite commonly used and it's
 // much easier to refer to everything through one crate rather than importing