MathML reader:  use styled identifier, not EText, for mi w/ mathvariant.

This seems more semantically correct, and agrees with how the TeXMath
reader works.
Moved getOperator from TeXMath writer to Shared.
TeXMath writer: Improved writing of identifiers.

We no longer put a \mathoperator{} around everything with
more than one token.

Also, don't print "invisible operators" from MathML.
Updated tests.