wasmtime: Add support for func.ref and table.grow with funcrefs

`funcref`s are implemented as `NonNull<VMCallerCheckedAnyfunc>`.

This should be more efficient than using a `VMExternRef` that points at a
`VMCallerCheckedAnyfunc` because it gets rid of an indirection, dynamic
allocation, and some reference counting.

Note that the null function reference is *NOT* a null pointer; it is a
`VMCallerCheckedAnyfunc` that has a null `func_ptr` member.

Part of #929

tests/misc_testsuite/reference-types/simple_ref_is_null.wast

@@ -0,0 +1,17 @@
+(module
+  (func (export "func_is_null") (param funcref) (result i32)
+    (ref.is_null func (local.get 0))
+  )
+  (func (export "func_is_null_with_non_null_funcref") (result i32)
+    (call 0 (ref.func 0))
+  )
+  (func (export "extern_is_null") (param externref) (result i32)
+    (ref.is_null extern (local.get 0))
+  )
+)
+
+(assert_return (invoke "func_is_null" (ref.null func)) (i32.const 1))
+(assert_return (invoke "func_is_null_with_non_null_funcref") (i32.const 0))
+
+(assert_return (invoke "extern_is_null" (ref.null extern)) (i32.const 1))
+(assert_return (invoke "extern_is_null" (ref.extern 1)) (i32.const 0))

tests/misc_testsuite/reference-types/table_grow_with_funcref.wast

@@ -0,0 +1,13 @@
+(module
+  (table $t 0 funcref)
+  (func (export "size") (result i32)
+    (table.size $t)
+  )
+  (func $f (export "grow-by-1") (result i32)
+    (table.grow $t (ref.func $f) (i32.const 1))
+  )
+)
+
+(assert_return (invoke "size") (i32.const 0))
+(assert_return (invoke "grow-by-1") (i32.const 0))
+(assert_return (invoke "size") (i32.const 1))