Add a concept of "global exports".

This adds a feature which allows one to look up an export by name
without knowing what module it's in -- `lookup_global_export` on an
`InstanceContents`.

The main expected use for this is to support APIs where module A
imports a function from module B, and module B needs to access module
A's memory. B can't import it from A in the normal way, because that
would create a dependency cycle. So for now, allow B to look up A's
exported memory dynamically with `lookup_global_export`.

In the future, with reference types and possibly host bindings, we'll be
able to pass references to memory as arguments, which will obviate the
need for this mechanism.

fuzz/fuzz_targets/compile.rs

@@ -9,6 +9,9 @@ extern crate wasmtime_environ;
 extern crate wasmtime_jit;
 
 use cranelift_codegen::settings;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
 use wasmparser::validate;
 use wasmtime_jit::{CompiledModule, Compiler, NullResolver};
 
@@ -23,7 +26,8 @@ fuzz_target!(|data: &[u8]| {
     let isa = isa_builder.finish(settings::Flags::new(flag_builder));
     let mut compiler = Compiler::new(isa);
     let mut resolver = NullResolver {};
-    let _compiled = match CompiledModule::new(&mut compiler, data, &mut resolver) {
+    let mut global_exports = Rc::new(RefCell::new(HashMap::new()));
+    let _compiled = match CompiledModule::new(&mut compiler, data, &mut resolver, global_exports) {
         Ok(x) => x,
         Err(_) => return,
     };