wasmtime: Implement table.get and table.set

These instructions have fast, inline JIT paths for the common cases, and only
call out to host VM functions for the slow paths. This required some changes to
`cranelift-wasm`'s `FuncEnvironment`: instead of taking a `FuncCursor` to insert
an instruction sequence within the current basic block,
`FuncEnvironment::translate_table_{get,set}` now take a `&mut FunctionBuilder`
so that they can create whole new basic blocks. This is necessary for
implementing GC read/write barriers that involve branching (e.g. checking for
null, or whether a store buffer is at capacity).

Furthermore, it required that the `load`, `load_complex`, and `store`
instructions handle loading and storing through an `r{32,64}` rather than just
`i{32,64}` addresses. This involved making `r{32,64}` types acceptable
instantiations of the `iAddr` type variable, plus a few new instruction
encodings.

Part of #929

crates/runtime/src/vmcontext.rs

@@ -555,6 +555,10 @@ impl VMBuiltinFunctionsArray {
             wasmtime_imported_memory_fill as usize;
         ptrs[BuiltinFunctionIndex::memory_init().index() as usize] = wasmtime_memory_init as usize;
         ptrs[BuiltinFunctionIndex::data_drop().index() as usize] = wasmtime_data_drop as usize;
+        ptrs[BuiltinFunctionIndex::drop_externref().index() as usize] =
+            wasmtime_drop_externref as usize;
+        ptrs[BuiltinFunctionIndex::activations_table_insert_with_gc().index() as usize] =
+            wasmtime_activations_table_insert_with_gc as usize;
 
         if cfg!(debug_assertions) {
             for i in 0..ptrs.len() {