Remove prototype of removed function
Check for new math font instead of space factor

OpenType math fonts have non-zero space unlike TFM math fonts, so the
check space(cur_f) ≠ 0 is always true.

http://tex.stackexchange.com/q/69632/729
Sync with TeX Live SVN r28043
Sync with TeX Live SVN r28043
Remove autogenerated files
Use \U prefix for extended math primitives

This way XeTeX and LuaTeX use the same prefixes for common primitives,
\XeTeX* is still accepted but might be deprecated and eventually removed
in the future.
Merge new xetex.web from TeX Live

Change files have been merged with tex.web, and we now have a canonical
xetex.web to hack on, no more change files complexity.
Redo 23f4bd3 on top of previous commit
Fix weaving xetex.web
Fix Texing the weaved file

Mostly commented out code blocks that TeX chocked at.
Add copyright and license header to xetex.web
Drop some trivial diffs
Cosmetics
Unused code
Cosmetics
Remove unused section
Remove outdated comment
Bin unused code
Start porting OT layout to HarfBuzz

ICU LayoutEngine we use now is essentially unmaintained and has many
long standing bugs. It will eventually switch to HarfBuzz internally,
but we are better using HarfBuzz directly to get access to its rich API.
Drop XeTeXOTLayoutEngine

Not used anymore (it was subclassing ICU LayoutEngine).