Fixes #935; fixes #936; fixes #938; fixes #939; fixes #941:

• add ascii, format and memoryview function builtins
• add a bunch of missing exception builtins
• add nonlocal keyword
• fix float lexer, add support for complex number literals
• add support for binary and octal numeric integer literals
• add support for capital letters in binary/octal/hex integer literals
• add support for underscores in numeric literals (PEP 515)
• dot followed by a numeric literal is not tagged as an operator
• support @ and @= operators (PEP 465)
• add //, ** and all variants of .= operators (like +=)

// Funnily enough, GitHub's lexer also fails at some of these (like 0_1.).