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/jit/src/debug.rs

@@ -27,9 +27,6 @@ pub fn create_gdbjit_image(
         }
     }
 
-    // let mut file = ::std::fs::File::create(::std::path::Path::new("test.o")).expect("file");
-    // ::std::io::Write::write_all(&mut file, &bytes).expect("write");
-
     Ok(bytes)
 }