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/runtime/src/vmcontext.rs

@@ -595,7 +595,7 @@ impl VMInvokeArgument {
 ///
 /// TODO: We could move the globals into the `vmctx` allocation too.
 #[derive(Debug)]
-#[repr(C)]
+#[repr(C, align(16))] // align 16 since globals are aligned to that and contained inside
 pub struct VMContext {}
 
 impl VMContext {
@@ -617,12 +617,4 @@ impl VMContext {
     pub unsafe fn host_state(&mut self) -> &mut dyn Any {
         self.instance().host_state()
     }
-
-    /// Lookup an export in the global exports namespace.
-    /// # Safety
-    /// This is unsafe because it doesn't work on just any `VMContext`, it must
-    /// be a `VMContext` allocated as part of an `Instance`.
-    pub unsafe fn lookup_global_export(&mut self, field: &str) -> Option<crate::export::Export> {
-        self.instance().lookup_global_export(field)
-    }
 }