Refactor how signatures/trampolines are stored in Store

This commit refactors where trampolines and signature information is
stored within a `Store`, namely moving them from
`wasmtime_runtime::Instance` instead to `Store` itself. The goal here is
to remove an allocation inside of an `Instance` and make them a bit
cheaper to create. Additionally this should open up future possibilities
like not creating duplicate trampolines for signatures already in the
`Store` when using `Func::new`.

crates/runtime/src/vmcontext.rs

@@ -492,6 +492,11 @@ impl VMSharedSignatureIndex {
     pub fn new(value: u32) -> Self {
         Self(value)
     }
+
+    /// Returns the underlying bits of the index.
+    pub fn bits(&self) -> u32 {
+        self.0
+    }
 }
 
 impl Default for VMSharedSignatureIndex {