Add delimiter sizing

Summary:
Make delimiter sizing work. This involved
 - Adding the symbols for the remaining delimiters (like `\lfloor` and `\{`)
 - Adding metrics for the size1, size2, size3, and size4 fonts
 - Parsing delimiter sizing functions
 - Using the big fonts when possible, otherwise building large copies of the
   delimiters from scratch

Test Plan:
 - See that
   `\bigl\uparrow\Bigl\downarrow\biggl\updownarrow\Biggl\Uparrow
    \Biggr\Downarrow\biggr\Updownarrow\bigm/\Bigm\backslash\biggm|
    \Biggm|\big\lceil\Big\rceil\bigg\langle\Bigg\rangle\bigl(\Bigl)
    \biggl[\Biggl]\Biggr\{\biggr\}\Bigr\lfloor\bigr\rfloor`
   parses correctly (this contains all of the delimiters, and all of the sizing
   modes)
 - See that the huxley tests didn't change, and the new one looks good
 - See the normal tests work

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7844