v0.7.0: vec2 and mat2 update

New Features:

• Now vec2 and mat2 are officially supported (including struct api) 🎉 ( #97 )
• add forward vector as macro (vec3, RH)

Bugfixes and Improvements:

• quat: fix quat_for and quat_forp implementations; These are identical to QuaternionLook / LookRotation in other frameworks/engines...
• add notes for versor/quaternion type to describe memory layout
• merge test sources into one C source
• make plane zero if norm is zero after normalization
• Add more tests

Since vec2 and mat2 are new features, bugs may occurs. Any feedback or help would be appreciated.