Use object-oriented style for trait methods

Wherever sensible the geometric traits now take `&self` arguments, so that
implementations can be invoked as instance methods instead of static methods.
This makes some idioms easier to express and generally allows for greater
flexibility as the restructured methods can still be invoked like static
methods.

Fixes #39.
Merge pull request #51 from aepsil0n/oop_style_traits

Use object-oriented style for trait methods