function log2( x ) {
	return addon( x );
}
Evaluates the binary logarithm.

@private
@param {number} x - input value
@returns {number} binary logarithm

@example
var v = log2( 4.0 );
// returns 2.0

v = log2( 8.0 );
// returns 3.0

v = log2( 0.0 );
// returns -Infinity

v = log2( Infinity );
// returns Infinity

v = log2( NaN );
// returns NaN