Switch from Scheme to Python; initial commit

Beginnings of lexer reimplementation in Python
Implement token classes and misc char predicates
Implement basic word tokenization
Switch from str to bytes
Implement operator tokenization
Implement double-quote processing for words
Add doctests for WordToken.from_stream and fix errors
Add doctests for OperatorToken
Implement doctests for single and double quotes
input_stream's type IOBase -> RawIOBase

io.IOBase doesn't declare a read method, which is used by Token and its subclasses.
Add support for backslashes in word tokens
Fix handling of line continuations
Token.from_stream -> Token._from_stream_pre

Token.from_stream shouldn't be public, nor should it return a Token. We should avoid changing __class__ directly if it can be helped.
Add copyright information