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/cranelift/src/obj.rs

@@ -37,7 +37,7 @@ use std::convert::TryFrom;
 use std::ops::Range;
 use wasmtime_environ::obj;
 use wasmtime_environ::{
-    DefinedFuncIndex, EntityRef, FuncIndex, Module, PrimaryMap, SignatureIndex,
+    DefinedFuncIndex, EntityRef, FuncIndex, Module, PrimaryMap, SignatureIndex, Trampoline,
 };
 
 const TEXT_SECTION_NAME: &[u8] = b".text";
@@ -230,9 +230,14 @@ impl<'a> ObjectBuilder<'a> {
         range
     }
 
-    pub fn trampoline(&mut self, sig: SignatureIndex, func: &'a CompiledFunction) {
+    pub fn trampoline(&mut self, sig: SignatureIndex, func: &'a CompiledFunction) -> Trampoline {
         let name = obj::trampoline_symbol_name(sig);
-        self.append_func(name.into_bytes(), func);
+        let (_, range) = self.append_func(name.into_bytes(), func);
+        Trampoline {
+            signature: sig,
+            start: range.start,
+            length: u32::try_from(range.end - range.start).unwrap(),
+        }
     }
 
     pub fn align_text_to(&mut self, align: u64) {