Custom Functions backed by algorithms

[stefnotch]

Some functions are best implemented as actual, programmatic algorithms (warning: XSS and more applies here).

For example, a number parser function parse(0x34) would best be done with actual code. It'd be lovely if we eventually could support defining such functions.

Relevant sympy custom functions documentation: https://docs.sympy.org/latest/modules/functions/index.html

[phcreery]

I agree, I like the idea of providing some fundamental or sophisticated yet necessary internal functions.