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/resolver.rs

@@ -5,15 +5,22 @@ use wasmtime_runtime::Export;
 
 /// Import resolver connects imports with available exported values.
 pub trait Resolver {
-    /// Resolve the given module/field combo.
-    fn resolve(&mut self, module: &str, field: &str) -> Option<Export>;
+    /// Resolves an import a WebAssembly module to an export it's hooked up to.
+    ///
+    /// The `index` provided is the index of the import in the wasm module
+    /// that's being resolved. For example 1 means that it's the second import
+    /// listed in the wasm module.
+    ///
+    /// The `module` and `field` arguments provided are the module/field names
+    /// listed on the import itself.
+    fn resolve(&mut self, index: u32, module: &str, field: &str) -> Option<Export>;
 }
 
 /// `Resolver` implementation that always resolves to `None`.
 pub struct NullResolver {}
 
 impl Resolver for NullResolver {
-    fn resolve(&mut self, _module: &str, _field: &str) -> Option<Export> {
+    fn resolve(&mut self, _idx: u32, _module: &str, _field: &str) -> Option<Export> {
         None
     }
 }