Reject invalid function names

[nex3]

From sass/sass#1598:

Sass has special parse rules for functions named calc, element, expression, and url that mean that user-defined functions with those names can never be run. We should emit an error when those functions are defined rather than allowing the user to define them and be unable to use them, as in sass/sass#1571.

calc and element may also have vendor prefixes.

Note that this is currently deprecated, and just needs to become an error on the master branch.