Replace the global-exports mechanism with a caller-vmctx mechanism. (#789)

* Replace the global-exports mechanism with a caller-vmctx mechanism.

This eliminates the global exports mechanism, and instead adds a
caller-vmctx argument to wasm functions so that WASI can obtain the
memory and other things from the caller rather than looking them up in a
global registry.

This replaces #390.

* Fixup some merge conflicts

* Rustfmt

* Ensure VMContext is aligned to 16 bytes

With the removal of `global_exports` it "just so happens" that this
isn't happening naturally any more.

* Fixup some bugs with double vmctx in wasmtime crate

* Trampoline stub needed adjusting
* Use pointer type instead of always using I64 for caller vmctx
* Don't store `ir::Signature` in `Func` since we don't know the pointer
  size at creation time.
* Skip the first 2 arguments in IR signatures since that's the two vmctx
  parameters.

* Update cranelift to 0.56.0

* Handle more merge conflicts

* Rustfmt

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

crates/api/src/runtime.rs

@@ -342,7 +342,6 @@ pub struct Store {
 struct StoreInner {
     engine: Engine,
     compiler: RefCell<Compiler>,
-    global_exports: Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>>,
     signature_cache: RefCell<HashMap<wasmtime_runtime::VMSharedSignatureIndex, ir::Signature>>,
 }
 
@@ -355,7 +354,6 @@ impl Store {
             inner: Rc::new(StoreInner {
                 engine: engine.clone(),
                 compiler: RefCell::new(compiler),
-                global_exports: Rc::new(RefCell::new(HashMap::new())),
                 signature_cache: RefCell::new(HashMap::new()),
             }),
         }
@@ -370,14 +368,6 @@ impl Store {
         self.inner.compiler.borrow_mut()
     }
 
-    // Specific to wasmtime: hack to pass memory around to wasi
-    #[doc(hidden)]
-    pub fn global_exports(
-        &self,
-    ) -> &Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>> {
-        &self.inner.global_exports
-    }
-
     pub(crate) fn register_wasmtime_signature(
         &self,
         signature: &ir::Signature,