Parse function definitions only once

[ArneBachmann]

Always parse (nested) function definitions only once, keeping them in a static immutable table.
This could also be the starting point for a clean way to avoid or allow function override in a controlled manner, and lead to a speed-up.

Related is the handling of function references, that currently only contain the function's name, but could point to a table index instead for different override versions of the same function name.

[ArneBachmann]

Requires a new runtime token for compiled function references (vs. reference on stack with apply).

[ArneBachmann]

Same could be used for static lists (currently there are no dynamic "literal" lists)