AHRS: perf improvement by caching declination vector
AP_Math: added mulXY() for matrix3 and vector3

returns the XY components of the product
AP_Math: implement vector2 cross product

useful in DCM for faster yaw correction
AP_AHRS: optimise yaw drift correction

use new vector2 cross product, and factor out the z component of a dcm
mul_transpose() to reduce the number of floating point operations for
a yaw drift correction cycle