Deparsing ALTER TABLE

[JackDanger]

I've done a couple things today:

• Began extracting some of the complex parts of the deparser into lib/deparser/{filename}.rb (just the ALTER TABLE and INTERVAL stuff now, because that's the most complex).
• Added support for deparsing ALTER TABLE statements and all of the various clauses that can appear with in that.

Tests included for everything, I'm keeping the tests like you set them up, @lfittl, so they just round-robin through the parser and deparser. This means anybody can refactor to their hearts content if they don't like the structure of the code :)

[lfittl]

Awesome - great work!

Will go through these changes in detail but looks good from a first look :)

[JackDanger]

I cleaned up the commits a bit. I apologize in advance for the size of this PR, it includes slightly more change than I'd normally put into one. I made it pretty straightforward to review commit-by-commit so that should make things easier. Most of the changes are actually just moved files or having to indent whole files to appease Rubocop.

[lfittl]

👍 Lets merge it in!