Add support for forest syntax trees

[omentic]

This is a companion PR to #55. Discussion should probably happen there. I can merge these together if desired.

[tarleb]

LGTM, but we should probably factor out the common parts of the tikz, forrest tree, and bussproof compilers. The main difference seems to be the LaTeX template that's used for each. Are there other differences?

[omentic]

The forest one puts \begin{forest}...\end{forest} in the template itself, so the user only needs to type ex. [S [N [Alice]] [V [runs]]] to get a syntax tree. The buffproofs one doesn't do this (like TikZ) since the syntax is more directly LaTeX.