Sync up a bit with gl-matrix

[rreusser]

See: stackgl/gl-vec2#3

Same story. First add tests, then some functions.

• sub (Add functions #3)
• mul (Add functions #3)
• div (Add functions #3)
• dist (Add functions #3)
• sqrDist (Add functions #3)
• len (Add functions #3)
• sqrLen (Add functions #3)
• forEach (Add functions #3)
• ceil (Add functions #3)
• floor (Add functions #3)
• round (Add functions #3)
• exactEquals (Add functions #3)
• equals (Add functions #3)
• str