function tanh( x ) {
	return addon( x );
}
Computes the hyperbolic tangent of a double-precision floating-point number.

@private
@param {number} x - input value
@returns {number} hyperbolic tangent

@example
var v = tanh( 0.0 );
// returns 0.0

@example
var v = tanh( 2.0 );
// returns ~0.964

@example
var v = tanh( -2.0 );
// returns ~-0.964

@example
var v = tanh( NaN );
// returns NaN