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

tests/misc_testsuite/reference-types/many_table_gets_lead_to_gc.wast

@@ -0,0 +1,35 @@
+(module
+  (table $t 1 externref)
+
+  (func (export "init") (param externref)
+    (table.set $t (i32.const 0) (local.get 0))
+  )
+
+  (func (export "get-many-externrefs") (param $i i32)
+    (loop $continue
+      ;; Exit when our loop counter `$i` reaches zero.
+      (if (i32.eqz (local.get $i))
+        (return)
+      )
+
+      ;; Get an `externref` out of the table. This could cause the
+      ;; `VMExternRefActivationsTable`'s bump region to reach full capacity,
+      ;; which triggers a GC.
+      ;;
+      ;; Set the table element back into the table, just so that the element is
+      ;; still considered live at the time of the `table.get`, it ends up in the
+      ;; stack map, and we poke more of our GC bits.
+      (table.set $t (i32.const 0) (table.get $t (i32.const 0)))
+
+      ;; Decrement our loop counter `$i`.
+      (local.set $i (i32.sub (local.get $i) (i32.const 1)))
+
+      ;; Continue to the next loop iteration.
+      (br $continue)
+    )
+    unreachable
+  )
+)
+
+(invoke "init" (ref.extern 1))
+(invoke "get-many-externrefs" (i32.const 8192))