Functions and function calling needs a cleanup

[jonathanhogg]

The function code is a bit of a mess, particularly where it comes to a division between native Python functions and Flitter functions. This is what I think needs doing:

• debug() should just be a @context_func in functions.pyx and all special pleading for it removed
• move references into the Context class and make sample() a @context_func as well
• allow func to declare recursive functions – this should just be a case of adding the function name before the parameters onto lnames list before compiling the body and then pushing it onto the lnames stack as part of the calling convention
• box all Flitter functions as Python function-like objects – basically turn call_helper() into a callable class wrapper around func Programs
• make all function calling use the Python call convention – dump CallFast and have Call just use PyObject_CallObject() or PyObject_Call() depending on whether any kwargs have been popped off the stack
• let the normal Python call stack handle and throw recursion errors as necessary

An advantage of all of this is that it will allow for higher-order functions, like passing Flitter functions into native functions as arguments. It'll also let me Cython-optimise the, currently quite slow, sample() function.

This also paves the way for possibly allowing functions to be passed into the rendering pipeline as attributes of Nodes…