For #42 - Rename "Null" to "Undef".

Add test for .toString()
prefix vs. postfix increment and decrement

The return value of $foo++ is different than ++$foo.  Likewise, $foo--
vs. --$foo.  This commit adds support for differentiating the two modes
(prefix vs. postfix).
Make the interpreter a class instead of an object
Pass arguments into subroutines
Move interpreter utils into its own file
Add a util for walking ASTs
Walk the subroutine body for closed over variables
Support basic class declaration in the interpreter
Clarify the code portrayal in the comment
Add attributes for classes to the interpreter
Add method declaration to the interpreter
Update the range operator to work with the updated code
simpler if/else construct
s/prefix/is_prefix/ to indicate the boolean-ness
WIP/EXPERIMENTAL: Attempt to expose class instance to runtime

This is just enough to make tests pass. May not be the right approach
but just seeing where this can go.
Test method resolution more deeply
Make instantiation work
Add a generic method call test
Add integer addition to the interpreter
Add the operators in the bootstrap instead

This feels safer, as it's run once when everything is being put
together.