Rewrite selectors.py and add more tests

[smallnamespace]

Rewrite selectors completely:

• Get rid of the 'Forth-like' language in favor of representing selector DSL nodes directly as Python objects
• No need to linearize the AST (which is what the Merge, Unmerge, and MergeFilter ops essentially did)
• Python program counter and stack are used instead of an explicitly managed stack / PC, which is faster
• I benchmarked a 10-20% performance improvement in full game runs, will profile selector performance explicitly later

[smallnamespace]

@mischanix
Would appreciate your comments

[smallnamespace]

Profiling runs at #302

[jleclanche]

If you do a first commit that creates a dummy class EnumSelector, child of Selector, and move all the enum selectors to it, this will be easier to review :)

[smallnamespace]

@jleclanche

I went ahead and rewrote the history like you suggested. Btw, my initial version of this used the typing module from 3.5, and there is a backport available. If you're ok with it, we can either:

1. Move Python requirement to 3.5 (would require updating Travis)
2. Make a hard dependency on the backport in requirements.txt (probably not ideal going forward, since it always will pull from PyPI even when unneeded)
3. or migrate requirements.txt to setup.py instead, so we can version-conditional dependencies

I would prefer 3 since it's ultimately cleanest while not moving up requirements and potentially breaking people -- for example, Ubuntu trusty only goes up to 3.4 by default.

[smallnamespace]

Split out AttrSelector -> AttrValue as requested

[robert-nix]

Looks good to me.

[jleclanche]

Typing annotations: Why are most of them strings of objects?

[jleclanche]

Overall code looks good, mostly just minor styling issues. Thanks. This looks like a really great rewrite.

[smallnamespace]

Most of the typing annotations were changed once I pulled in the py3.5 commit.

The remaining ones which are strings are forward references so type hints on a class's methods can refer to the class type itself (which hasn't been constructed yet).

[jleclanche]

Landed your first 2 commits. Here's what's left to do:

1. Add an autoclose to the 3.5 commit
2. Sync your 3.5 PR with that commit so that it closes when I land this
3. Take care of the last remaining comments

[smallnamespace]

Done