Motivation:
We used to approximate s33/i33 value parsing by first checking for
specific negative codes, and then parsing an u32 value if that failed.
This is not correct in all cases.

Changes:
- Implement i33 parsing in Decoder.
- Factor out parsing of heap types into read_heap_type.
- Introduce HeapType::kBottom.
- Introduce helper functions in WasmFeatures and value_type_reader.
- Remove macros from the parsing of value types.
- HeapType::code now returns an i32 for compatibility with the i33
  requirement.
- Introduce HeapType::Repr.
- Renamings: HeapType::type() -> representation(),
             ValueType::heap() -> heap_representation()

Bug: v8:7748
Change-Id: I04deabce8837a48af2226411cd706a397f9e5725
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2274118
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68633}