Changes:
- Implement the 'let' opcode, as per
https://github.com/WebAssembly/function-references/blob/master/proposals/function-references/Overview.md#local-bindings
- Use a WasmDecoder in place of a plain decoder in OpcodeLength and
AnalyzeLoopAssignment.
- Change ControlBase to accept an additional 'locals_count' parameter.
- Implement required test infrastructure and write some simple tests.

Bug: v8:7748
Change-Id: I39d60d1f0c26016c8f89c009dc5f4119b0c73c87
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2204107
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67937}