Add a flag to FuncIR to allow functions to be marked as internal.  Don't
generate wrapper functions/methods that allow calls from Python for
internal methods, since these are internal implementation details.

This has these benefits:
 * Internal functions are private and don't pollute public namespaces.
 * Signatures of generated functions can use arbitrary C types
   (e.g. arbitrary pointer types), even those that can't be passed
   to/from Python.
 * We generate less C code (fewer wrapper functions).