Handle same-named imports with different signatures

This commit fixes the `wasmtime::Instance` instantiation API when
imports have the same name but might be imported under different types.
This is handled in the API by listing imports as a list instead of as a
name map, but they were interpreted as a name map under the hood causing
collisions.

This commit now keeps track of the index used to define each import, and
the index is passed through in the `Resolver`. Existing implementaitons
of `Resolver` all ignore this, but the API now uses it exclusivley to
match up `Extern` definitions to imports.

crates/api/src/externals.rs

@@ -56,7 +56,7 @@ impl Extern {
         }
     }
 
-    pub(crate) fn get_wasmtime_export(&mut self) -> wasmtime_runtime::Export {
+    pub(crate) fn get_wasmtime_export(&self) -> wasmtime_runtime::Export {
         match self {
             Extern::Func(f) => f.borrow().wasmtime_export().clone(),
             Extern::Global(g) => g.borrow().wasmtime_export().clone(),