Implement use of keyword arguments for __init__ methods.
Pass through all argument types to klass, so they will work correctly
with __init__ if they are implemented later.
Fix type built-in function output to use 'type' or 'class' correctly.
Make isinstance throw TypeErrors like Python for invalid arguments.
Add sk$type to enable type checking on classes.

Rename $is_klass to be sk$klass.
rebuild skulpt