RFC: tab substitution of LaTeX symbols in REPL

This patch allows you to type things like `\alpha<TAB>` in the REPL to get `α` (as discussed in #6340).  You can even type e.g. `x\dot<TAB>` to get `ẋ`.

Currently, you have to type out the whole LaTeX symbol....it seemed too complicated (at least in a first implementation) to support a two-level completion scheme where you could get a list of LaTeX symbols with a given prefix.

The list of substitutions is automatically generated from the [W3C LaTeX/Unicode mapping](http://www.w3.org/Math/characters/unicode.xml).