Refactor the compilation and instantiation pipeline.

wasmtime-execute is now wasmtime-jit. Move `JITCode` and the TargetIsa
into a new `Compiler` type. `InstancePlus` is no more, with trampoline
functionality now handled by `Compiler`.

lib/jit/src/resolver.rs

@@ -0,0 +1,19 @@
+//! Define the `Resolver` trait, allowing custom resolution for external
+//! references.
+
+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>;
+}
+
+/// `Resolver` implementation that always resolves to `None`.
+pub struct NullResolver {}
+
+impl Resolver for NullResolver {
+    fn resolve(&mut self, _module: &str, _field: &str) -> Option<Export> {
+        None
+    }
+}