Remove commented out line of code

Create a new SourceLocation object for each Token

Fix flake8 errors

Switch to PEP8 79 column line length

Match idiomatic inline comment style

Add linting with flake8

Remove some boilerplate in parser tests

Move eval_ast closer to the other eval_* functions

Make internal eval functions private

Make helper Parser methods private

Make comment consistent

Change class REPL to function repl

Switch to more idomatic `is None` and `is not None`

Fix errors generated by mypy 0.670

Improve __str__ method for illegal tokens

Gracefully handle parse errors during batch mode

Add eval_file

Also adds the ability to run a source file from the command line with an
optional `file` argument to monkey.py.

Make SourceLocation optional in Lexer constructor

Make SourceLocation optional in Token constructor

Add tests for Token