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/wasi-common/wig/src/wasi.rs

@@ -193,6 +193,7 @@ pub fn add_wrappers_to_module(args: TokenStream) -> TokenStream {
 
                 unsafe extern "C" fn #name_ident(
                     ctx: *mut wasmtime_runtime::VMContext,
+                    caller_ctx: *mut wasmtime_runtime::VMContext,
                     #(#shim_arg_decls),*
                 ) -> #ret_ty {
                     log::trace!(
@@ -203,7 +204,7 @@ pub fn add_wrappers_to_module(args: TokenStream) -> TokenStream {
                         Ok(e) => e,
                         Err(e) => #handle_early_error,
                     };
-                    let memory = match get_memory(&mut *ctx) {
+                    let memory = match get_memory(&mut *caller_ctx) {
                         Ok(e) => e,
                         Err(e) => #handle_early_error,
                     };