Replace manual bit flags with auto-generated bit flag construct

Created by: huntergoldstein

Problem: There are small ergonomics issues with using hand-rolled bit flags as described in #308 (closed).

Solution: Replace the use of the hand-rolled flags with auto-generated bitflags structures that provide nice mnemonics for membership while being mostly compatible with the old syntax.

Changes introduced by this pull request:

• Bitflags have been replaced with bitflag! equivalents as per the bitflag crate

Drawbacks:

• I'm not entirely sure if this is any more readable than before: part of the problem is contains versus intersects, where foo.contains(bar) is foo & bar == bar and foo.intersects(bar) is foo & bar != 0. These names are non-intuitive.
• In addition to the above, the operators tended to be much more concise so for the most part I tried to not replace them with the mnemonics.
• There is a slight chance that this is overall less efficient (more function calls) but I highly doubt it; the bitflags crate essentially has #[inline] on every function

Fixes: Closes #308 (closed).

State: I'm on the fence on whether or not this is a worthwhile addition, but I have nothing else to add.

Thanks