Add support for \left and \right

Summary:
Added stacked delimiter support for more delimiters. Split out delimiter
functions into its own file, and split out some tree building functions into a
common file. Supports the empty `.` delimiter with \left and \right, and doesn't
try to produce huge /, \backslash, <, or > delimiters. Depends on D7844

Test input:

\left( \left) \left[ \left\lbrack \left] \left\rbrack \left\{ \left\lbrace
\left\} \left\rbrace \left\lfloor \left\rfloor \left\lceil \left\rceil
\left\langle \left\rangle \left/ \left\backslash \left| \left\vert \left\|
\left\Vert \left\uparrow \left\Uparrow \left\downarrow \left\Downarrow
\left\updownarrow \left\Updownarrow {x^{x^{x^{x^{x^{x^{x^{x^{x^{x^x}}}}}}}}}}
\right.\right.\right.\right.\right.\right.\right.\right.\right.\right.
\right.\right.\right.\right.\right.\right.\right.\right.\right.\right.
\right.\right.\right.\right.\right.\right.\right.\right.

Test Plan:
 - Run the test input, see that it works
 - Run the tests, see that they work
 - Look at huxley screenshots (not here yet :( ) and make sure they look good

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D11602