Fixes several issues with space handling: (fix#910)
1. "Control symbols" (as they're called in the TeXbook), such as `\\`, should
not have spaces eaten after them (only "control words" such as `\foo`).
2. In math mode, spaces should be consumed at the parser level, not the
gullet level. This enables `\\ [x]` to parse differently from `\\[x]`
3. Eat spaces between arguments, so `\frac x y` still works.
(This used to work only because math mode ate all spaces.
The analog in text mode wouldn't have worked.)
Also eat spaces in initial arguments in math mode, and before ^ and _ in atoms.
3280652bd6