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.
41780fb1a6