Code review feedback.

* Make `FunctionInfo` public and `CompiledModule::func_info` return it.
* Make the `StackMapLookup` trait unsafe.
* Add comments for the purpose of `EngineHostFuncs`.
* Rework ownership model of shared signatures: `SignatureCollection` in
  conjunction with `SignatureRegistry` is now used so that the `Engine`,
  `Store`, and `Module` don't need to worry about unregistering shared
  signatures.
* Implement `Func::param_arity` and `Func::result_arity` in terms of
  `Func::ty`.
* Make looking up a trampoline with the module registry more efficient by doing
  a binary search on the function's starting PC value for the owning module and
  then looking up the trampoline with only that module.
* Remove reference to the shared signatures from `GlobalRegisteredModule`.

crates/runtime/src/externref.rs

@@ -742,14 +742,22 @@ impl VMExternRefActivationsTable {
 }
 
 /// Used by the runtime to lookup a stack map from a PC value.
-pub trait StackMapLookup {
+///
+/// # Safety
+///
+/// This trait is unsafe as it returns pointers to a stack map without
+/// any clear ownership.
+///
+/// It is the responsibility of the caller to not have the pointer outlive
+/// the stack map lookup trait object.
+pub unsafe trait StackMapLookup {
     /// Lookup the stack map at a program counter (PC) value.
     fn lookup(&self, pc: usize) -> Option<*const StackMap>;
 }
 
 pub(crate) struct EmptyStackMapLookup;
 
-impl StackMapLookup for EmptyStackMapLookup {
+unsafe impl StackMapLookup for EmptyStackMapLookup {
     fn lookup(&self, _pc: usize) -> Option<*const StackMap> {
         None
     }