TeXMath Writer: EStretchy is reinterpreted as \\middle

This might need some tweaking and changes to other writers.
MathML Reader: Now uses EDelimited to match braces
Various: Removing EStretchy from source
TeXMath reader:  removed undefined in Delimited, added 'fence' helper.
Removed EStretchy from Types.
TeXMath Writer: Improved delimiter rendering
Merge branch 'delimited' of https://github.com/mpickering/texmath into mpickering-delimited

Conflicts:
	src/Text/TeXMath/Writers/TeXMath.hs
Merge branch 'mpickering-delimited'
Updated tests.