Splits shared/annotation.js into core/ and display/
Add ChunkedStream.ensureByte().

This new function is much faster than ensureRange(pos, pos+1), which is a very
common case.

This speeds up the rendering of some test cases (including the Tracemonkey
paper) by 4--5%.
Merge pull request #4966 from nnethercote/faster-ensureRange-2

Add ChunkedStream.ensureByte().
Merge pull request #4965 from yurydelendik/annotations

Splits shared/annotation.js into core/ and display/