- Smaller, more consistent streams API (Advance, Back, pos, Seek)
- Remove implementations from the header, in favor of creation functions.

Observe:
- Performance:
  - All Utf16CharacterStream methods have an inlinable V8_LIKELY w/ a
    body of only a few instructions. I expect most calls to end up there.
  - There used to be performance problems w/ bookmarking, particularly
    with copying too much data on SetBookmark w/ UTF-8 streaming streams.
    All those copies are gone.
  - The old streaming streams implementation used to copy data even for
    2-byte input. It no longer does.
  - The only remaining 'slow' method is the Seek(.) slow case for utf-8
    streaming streams. I don't expect this to be called a lot; and even if,
    I expect it to be offset by the gains in the (vastly more frequent)
    calls to the other methods or the 'fast path'.
  - If it still bothers us, there are several ways to speed it up.
- API & code cleanliness:
  - I want to remove the 'old' API in a follow-up CL, which should mostly
    delete code, or replace it 1:1.
  - In a 2nd follow-up I want to delete much of the UTF-8 handling in Blink
    for streaming streams.
  - The "bookmark" is now always implemented (and mostly very fast), so we
    should be able to use it for more things.
- Testing & correctness:
  - The unit tests now cover all stream implementations,
    and are pretty good and triggering all the edge cases.
  - Vastly more DCHECKs of the invariants.

BUG=v8:4947

Review-Url: https://codereview.chromium.org/2314663002
Cr-Commit-Position: refs/heads/master@{#39464}