- Removes Utf8Iterator
- Replaces Utf8Decoder with something based on ValueOfIncremental +
  NonAsciiStart and moves it into v8/internal.
- Internalizes utf8 strings by first converting them to one or two byte
- Removes IsUtf8EqualsTo and replaces current uses with IsOneByteEqualsTo

Tbr: jgruber@chromium.org
Change-Id: I16e08d910a745e78d6fd465718fc69ad731fd217
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1585840
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61049}