Added Text.TeXMath.TeX (unexported) with TeX defn.

Removed from Writers.TeXMath.
Make getTeXMath return [TeX] instead of String.

Also restored addition of brackets after certain commands
(e.g. accent commands).
Don't write identifiers as operators.

Cleans up a weird bug I introduced in an earlier commit.
Improved treatment of operators and identifiers.
Fixed bugs introduced by an ugly rebase.
Improve EText rendering.

Moved escapeLaTeX to TeX module.
texmath: Re-removed environment delimiters.
TeXMath writer:  Better handling of single-character identifiers.
Fixed order of overset and underset.