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/module.rs

@@ -3,7 +3,7 @@
 use crate::{EntityRef, ModuleTranslation, PrimaryMap, Tunables, WASM_PAGE_SIZE};
 use indexmap::IndexMap;
 use serde::{Deserialize, Serialize};
-use std::collections::{BTreeMap, BTreeSet};
+use std::collections::BTreeMap;
 use std::convert::TryFrom;
 use std::ops::Range;
 use wasmtime_types::*;
@@ -422,10 +422,6 @@ pub struct Module {
 
     /// The type of each nested wasm module this module contains.
     pub modules: PrimaryMap<ModuleIndex, ModuleTypeIndex>,
-
-    /// The set of defined functions within this module which are located in
-    /// element segments.
-    pub possibly_exported_funcs: BTreeSet<DefinedFuncIndex>,
 }
 
 /// Initialization routines for creating an instance, encompassing imports,