Fully support multiple returns in Wasmtime (#2806)

* Fully support multiple returns in Wasmtime

For quite some time now Wasmtime has "supported" multiple return values,
but only in the mose bare bones ways. Up until recently you couldn't get
a typed version of functions with multiple return values, and never have
you been able to use `Func::wrap` with functions that return multiple
values. Even recently where `Func::typed` can call functions that return
multiple values it uses a double-indirection by calling a trampoline
which calls the real function.

The underlying reason for this lack of support is that cranelift's ABI
for returning multiple values is not possible to write in Rust. For
example if a wasm function returns two `i32` values there is no Rust (or
C!) function you can write to correspond to that. This commit, however
fixes that.

This commit adds two new ABIs to Cranelift: `WasmtimeSystemV` and
`WasmtimeFastcall`. The intention is that these Wasmtime-specific ABIs
match their corresponding ABI (e.g. `SystemV` or `WindowsFastcall`) for
everything *except* how multiple values are returned. For multiple
return values we simply define our own version of the ABI which Wasmtime
implements, which is that for N return values the first is returned as
if the function only returned that and the latter N-1 return values are
returned via an out-ptr that's the last parameter to the function.

These custom ABIs provides the ability for Wasmtime to bind these in
Rust meaning that `Func::wrap` can now wrap functions that return
multiple values and `Func::typed` no longer uses trampolines when
calling functions that return multiple values. Although there's lots of
internal changes there's no actual changes in the API surface area of
Wasmtime, just a few more impls of more public traits which means that
more types are supported in more places!

Another change made with this PR is a consolidation of how the ABI of
each function in a wasm module is selected. The native `SystemV` ABI,
for example, is more efficient at returning multiple values than the
wasmtime version of the ABI (since more things are in more registers).
To continue to take advantage of this Wasmtime will now classify some
functions in a wasm module with the "fast" ABI. Only functions that are
not reachable externally from the module are classified with the fast
ABI (e.g. those not exported, used in tables, or used with `ref.func`).
This should enable purely internal functions of modules to have a faster
calling convention than those which might be exposed to Wasmtime itself.

Closes #1178

* Tweak some names and add docs

* "fix" lightbeam compile

* Fix TODO with dummy environ

* Unwind info is a property of the target, not the ABI

* Remove lightbeam unused imports

* Attempt to fix arm64

* Document new ABIs aren't stable

* Fix filetests to use the right target

* Don't always do 64-bit stores with cranelift

This was overwriting upper bits when 32-bit registers were being stored
into return values, so fix the code inline to do a sized store instead
of one-size-fits-all store.

* At least get tests passing on the old backend

* Fix a typo

* Add some filetests with mixed abi calls

* Get `multi` example working

* Fix doctests on old x86 backend

* Add a mixture of wasmtime/system_v tests

crates/environ/src/module.rs

@@ -2,12 +2,11 @@
 
 use crate::tunables::Tunables;
 use crate::WASM_MAX_PAGES;
-use cranelift_codegen::ir;
 use cranelift_entity::{EntityRef, PrimaryMap};
 use cranelift_wasm::*;
 use indexmap::IndexMap;
 use serde::{Deserialize, Serialize};
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::sync::Arc;
 
 /// Implemenation styles for WebAssembly linear memory.
@@ -367,6 +366,10 @@ pub struct Module {
 
     /// The type of each nested wasm module this module contains.
     pub modules: PrimaryMap<ModuleIndex, ModuleTypeIndex>,
+
+    /// The set of defined functions within this module which are located in
+    /// element segments.
+    pub possibly_exported_funcs: HashSet<DefinedFuncIndex>,
 }
 
 /// Initialization routines for creating an instance, encompassing imports,
@@ -564,7 +567,6 @@ impl Module {
 #[allow(missing_docs)]
 pub struct TypeTables {
     pub wasm_signatures: PrimaryMap<SignatureIndex, WasmFuncType>,
-    pub native_signatures: PrimaryMap<SignatureIndex, ir::Signature>,
     pub module_signatures: PrimaryMap<ModuleTypeIndex, ModuleSignature>,
     pub instance_signatures: PrimaryMap<InstanceTypeIndex, InstanceSignature>,
 }