wasmtime: Refactor how imports are resolved (#2102)

This commit removes all import resolution handling from the
`wasmtime-jit` crate, instead moving the logic to the `wasmtime` crate.
Previously `wasmtime-jit` had a generic `Resolver` trait and would do
all the import type matching itself, but with the upcoming
module-linking implementation this is going to get much trickier.

The goal of this commit is to centralize all meaty "preparation" logic
for instantiation into one location, probably the `wasmtime` crate
itself. Instantiation will soon involve recursive instantiation and
management of alias definitions as well. Having everything in one
location, especially with access to `Store` so we can persist
instances for safety, will be quite convenient.

Additionally the `Resolver` trait isn't really necessary any more since
imports are, at the lowest level, provided as a list rather than a map
of some kind. More generic resolution functionality is provided via
`Linker` or user layers on top of `Instance::new` itself. This makes
matching up provided items to expected imports much easier as well.

Overall this is largely just moving code around, but most of the code
in the previous `resolve_imports` phase can be deleted since a lot of it
is handled by surrounding pieces of `wasmtime` as well.

crates/runtime/src/sig_registry.rs

@@ -50,6 +50,11 @@ impl SignatureRegistry {
         }
     }
 
+    /// Looks up a shared index from the wasm signature itself.
+    pub fn lookup(&self, wasm: &WasmFuncType) -> Option<VMSharedSignatureIndex> {
+        self.wasm2index.get(wasm).cloned()
+    }
+
     /// Looks up a shared native signature within this registry.
     ///
     /// Note that for this operation to be semantically correct the `idx` must