Fexl standard context now defined in terms of elegant combinators

The Fexl standard context is now defined very elegantly, in a way which scales very nicely even to thousands of built-in symbols.

In particular here's the definition of "enhance", which is used to enhance the standard context with a potentially enormous set of extra definitions, spread across multiple files:

https://github.com/chkoreff/Fexl/blob/ced168cae71644ff9a9cb3...