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/jit/src/action.rs

@@ -187,11 +187,13 @@ pub fn invoke(
     // Make all JIT code produced thus far executable.
     compiler.publish_compiled_code();
 
-    // Call the trampoline.
+    // Call the trampoline. Pass a null `caller_vmctx` argument as `invoke` is
+    // all about calling from the outside world rather than from an instance.
     if let Err(trap) = unsafe {
         instance.with_signals_on(|| {
             wasmtime_call_trampoline(
                 callee_vmctx,
+                ptr::null_mut(),
                 exec_code_buf,
                 values_vec.as_mut_ptr() as *mut u8,
             )