support many more Unicode infix operators

This patch adds support for a much larger number of infix operators, based on [my comment in #6582](https://github.com/JuliaLang/julia/pull/6582#issuecomment-41055809): I went through category Sm manually and pulled out a list of code points that seemed (a) unambiguously infix and (b) had a clear analogy to existing operators so that a reasonable precedence choice could be made.

I don't actually provide definitions for any of the new operators, but now they are available for the user to add methods if she wants to.

It also adds the synonym `∛` for `cbrt`, in analogy to `√` for `sqrt`.

cc: @jiahao, @JeffBezanson