Main changes:
- Allow global.get in elements segments with expressions-as-elements.
- Allow element segments with types other than funcref.

Detailed changes:
- Move WasmInitExpr to its own file. Add stream opearator << support.
- Simplify type of PrintCollection.
- Make WasmElemSegment use an array of WasmInitExpr's over the previous
  ad-hoc implementation. Move null_index to WasmModuleBuilder.
- Refactor consume_element_segment_header. Make it return a
  WasmElemSegment.
- Refactor consume_element_expr. Make it return a WasmInitExpr.
- Refactor DecodeElementSection. Make it invoke
  consume_element_segment_header, then populate its element array.
- Update module-instantiate.cc to handle global.get elements.
- Fix bug in wasm-objects.cc where the wrong type index was passed into
  module()->has_signature()
- Adapt and add tests.

Change-Id: I5abfbe424dbb750ee2dca59f91c451ffcb79f95f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2857959
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74374}