externref: implement stack map-based garbage collection

For host VM code, we use plain reference counting, where cloning increments
the reference count, and dropping decrements it. We can avoid many of the
on-stack increment/decrement operations that typically plague the
performance of reference counting via Rust's ownership and borrowing system.
Moving a `VMExternRef` avoids mutating its reference count, and borrowing it
either avoids the reference count increment or delays it until if/when the
`VMExternRef` is cloned.

When passing a `VMExternRef` into compiled Wasm code, we don't want to do
reference count mutations for every compiled `local.{get,set}`, nor for
every function call. Therefore, we use a variation of **deferred reference
counting**, where we only mutate reference counts when storing
`VMExternRef`s somewhere that outlives the activation: into a global or
table. Simultaneously, we over-approximate the set of `VMExternRef`s that
are inside Wasm function activations. Periodically, we walk the stack at GC
safe points, and use stack map information to precisely identify the set of
`VMExternRef`s inside Wasm activations. Then we take the difference between
this precise set and our over-approximation, and decrement the reference
count for each of the `VMExternRef`s that are in our over-approximation but
not in the precise set. Finally, the over-approximation is replaced with the
precise set.

The `VMExternRefActivationsTable` implements the over-approximized set of
`VMExternRef`s referenced by Wasm activations. Calling a Wasm function and
passing it a `VMExternRef` moves the `VMExternRef` into the table, and the
compiled Wasm function logically "borrows" the `VMExternRef` from the
table. Similarly, `global.get` and `table.get` operations clone the gotten
`VMExternRef` into the `VMExternRefActivationsTable` and then "borrow" the
reference out of the table.

When a `VMExternRef` is returned to host code from a Wasm function, the host
increments the reference count (because the reference is logically
"borrowed" from the `VMExternRefActivationsTable` and the reference count
from the table will be dropped at the next GC).

For more general information on deferred reference counting, see *An
Examination of Deferred Reference Counting and Cycle Detection* by Quinane:
https://openresearch-repository.anu.edu.au/bitstream/1885/42030/2/hon-thesis.pdf

cc #929

Fixes #1804

src/obj.rs

@@ -99,19 +99,26 @@ pub fn compile_to_obj(
         .translate(wasm)
         .context("failed to translate module")?;
 
-    // TODO: use the traps information
-    let (compilation, relocations, address_transform, value_ranges, stack_slots, _traps) =
-        match strategy {
-            Strategy::Auto | Strategy::Cranelift => {
-                Cranelift::compile_module(&translation, &*isa, cache_config)
-            }
-            #[cfg(feature = "lightbeam")]
-            Strategy::Lightbeam => Lightbeam::compile_module(&translation, &*isa, cache_config),
-            #[cfg(not(feature = "lightbeam"))]
-            Strategy::Lightbeam => bail!("lightbeam support not enabled"),
-            other => bail!("unsupported compilation strategy {:?}", other),
+    // TODO: use the traps and stack maps information.
+    let (
+        compilation,
+        relocations,
+        address_transform,
+        value_ranges,
+        stack_slots,
+        _traps,
+        _stack_maps,
+    ) = match strategy {
+        Strategy::Auto | Strategy::Cranelift => {
+            Cranelift::compile_module(&translation, &*isa, cache_config)
         }
-        .context("failed to compile module")?;
+        #[cfg(feature = "lightbeam")]
+        Strategy::Lightbeam => Lightbeam::compile_module(&translation, &*isa, cache_config),
+        #[cfg(not(feature = "lightbeam"))]
+        Strategy::Lightbeam => bail!("lightbeam support not enabled"),
+        other => bail!("unsupported compilation strategy {:?}", other),
+    }
+    .context("failed to compile module")?;
 
     if compilation.is_empty() {
         bail!("no functions were found/compiled");