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/callable.rs

@@ -3,6 +3,7 @@ use crate::trampoline::{generate_func_export, take_api_trap};
 use crate::trap::Trap;
 use crate::types::FuncType;
 use crate::values::Val;
+use std::ptr;
 use std::rc::Rc;
 use wasmtime_environ::ir;
 use wasmtime_jit::InstanceHandle;
@@ -150,6 +151,7 @@ impl WrappedCallable for WasmtimeFn {
             self.instance.with_signals_on(|| {
                 wasmtime_runtime::wasmtime_call_trampoline(
                     vmctx,
+                    ptr::null_mut(),
                     exec_code_buf,
                     values_vec.as_mut_ptr() as *mut u8,
                 )