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/test-programs/tests/wasm_tests/runtime.rs

@@ -6,7 +6,6 @@ use wasmtime::{Instance, Module, Store};
 pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> anyhow::Result<()> {
     let store = Store::default();
 
-    let global_exports = store.global_exports().clone();
     let get_preopens = |workspace: Option<&Path>| -> anyhow::Result<Vec<_>> {
         if let Some(workspace) = workspace {
             let preopen_dir = wasi_common::preopen_dir(workspace)
@@ -37,7 +36,6 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
     let snapshot1 = Instance::from_handle(
         &store,
         wasmtime_wasi::instantiate_wasi_with_context(
-            global_exports.clone(),
             builder.build().context("failed to build wasi context")?,
         )
         .context("failed to instantiate wasi")?,