Fix stupid bug in to_mat3/4

Accidentally had the classes swapped for some unknown reason.
Fix mat3/4_rotation.

Output was .. correct but subtly incorrect. Only noticed this because
pitch/yaw values were negated for some reason, which looked correct in
some tests (single values) but when applied in a frame loop to simplify
entity tranasforms, it became very obvious something was seriously
wrong.
Fix mat3/mat4_from_quat.

Made some subtle mistakes in there-- mainly to do with how the scale-by-
-two commuted (badly). So, that's fixed and I feel stupid.
Add pitch/yaw/roll methods to Quat.
Fix swizzle docs mention of no vec2.
Version 1.7.1

- Adds Quat#pitch, #yaw, and #roll.
- Fixes Swizzle docs.
- Fixes some bugs in Mat3/Mat4 rotations and conversions from
  quaternions.
- Fixes to_mat3/mat4 returning the incorrect types. Not sure how that
  one happened.