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/jit/src/lib.rs

@@ -23,11 +23,9 @@
 
 mod code_memory;
 mod compiler;
-mod imports;
 mod instantiate;
 mod link;
 mod object;
-mod resolver;
 mod unwind;
 
 pub mod native;
@@ -37,7 +35,6 @@ pub use crate::code_memory::CodeMemory;
 pub use crate::compiler::{Compilation, CompilationStrategy, Compiler};
 pub use crate::instantiate::{CompilationArtifacts, CompiledModule, SetupError};
 pub use crate::link::link_module;
-pub use crate::resolver::{NullResolver, Resolver};
 
 /// Version number of this crate.
 pub const VERSION: &str = env!("CARGO_PKG_VERSION");