Latex backend: support for line breaking inside rewrites.
1. Eliminated some more parasytic spaces
2. Improved overal spacing between various term elements, by trying mathrel, mathord, mathpunct etc.
3. changed latex representation of builtins and some brackets, to improve line breaking support.

4. Implemented first version of line breaking inside a rewrite. Not ideal, but very good.

Remaining tasks:
- improve hspace of a long rewrite, based on variable environment spacing
- improve angle braces size determination. Right now the size is based on the LHS of the rewrite only.
- implement line breaking inside cells.