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/jit/src/namespace.rs

@@ -36,7 +36,7 @@ impl Namespace {
 }
 
 impl Resolver for Namespace {
-    fn resolve(&mut self, name: &str, field: &str) -> Option<Export> {
+    fn resolve(&mut self, _idx: u32, name: &str, field: &str) -> Option<Export> {
         if let Some(instance) = self.names.get_mut(name) {
             instance.lookup(field)
         } else {