We enable struct.new and array.init initializer expressions in the JS
testcase generated by --wasm-fuzzer-gen-test. We needed to make some
changes in the WasmInitExpr class, and to implement a new interface for
the WasmFullDecoder, which constructs a WasmInitExpr.
Changes:
- Make WasmInitExpr a ZoneObject. Use a pointer for its operands_ field.
  This is needed so WasmInitExpr is trivially copiable, and thus usable
  as a Value type in WasmFullDecoder.
- Implement a WasmFullDecoder interface in wasm-fuzzer-common that
  constructs a WasmInitExpr. Use it to decode initializers in the
  module generated by the fuzzer.
- Change AppendInitExpr to take a WasmInitExpr as argument.
- Fix an issue with printing of struct definitions.
- Change initializer expression used for structs to struct.new_with_rtt.
  This is consistent with the currently used structural types.

Bug: v8:11954
Change-Id: I65a87cc98701a54f32500be192b3b6eef2ff6c8c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3257712Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77730}