Extending AMSMath

So here we are again. This time, I only added to the building-function modules, without touching the `LaTeX` type constructors. Hope you're ok with it now.

Of course, this new `MathType = MathEnv | DispEnv | EqEnv | Parentheses | Square | Dollar`, as apparently introduced by Tobias Schoofs, in a way has the same problems as my first proposal: the first three constructors are redundant with `TeXEnv`, and though it's nice that they allow parsing math as math, rather than generic environments, they aren't complete anyway: there's also `begin{align}...\end{align}`, `\begin{dmath}...\end{dmath}`, etc. pp. – which you certainly would not want to include in the `Syntax` module. What is you opinion on this?