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/trampoline/create_handle.rs

@@ -3,8 +3,7 @@
 use crate::runtime::Store;
 use anyhow::Result;
 use std::any::Any;
-use std::cell::RefCell;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::rc::Rc;
 use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::wasm::DefinedFuncIndex;
@@ -17,9 +16,6 @@ pub(crate) fn create_handle(
     finished_functions: PrimaryMap<DefinedFuncIndex, *const VMFunctionBody>,
     state: Box<dyn Any>,
 ) -> Result<InstanceHandle> {
-    let global_exports: Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>> =
-        Rc::new(RefCell::new(HashMap::new()));
-
     let imports = Imports::new(
         HashSet::new(),
         PrimaryMap::new(),
@@ -42,7 +38,6 @@ pub(crate) fn create_handle(
 
     Ok(InstanceHandle::new(
         Rc::new(module),
-        global_exports,
         finished_functions.into_boxed_slice(),
         imports,
         &data_initializers,