function bitdiff( a, b ) {
	var astr;
	var bstr;
	var i;

	astr = toBinaryString( a );
	bstr = toBinaryString( b );
	for ( i = 0; i < 64; i++ ) {
		if ( astr[ i ] !== bstr[ i ] ) {
			return i;
		}
	}
	return -1;
}
Compares the binary representations of two double-precision floating-point numbers and returns the first index of a differing bit. If all bits match, the function returns `-1`.

TODO: revisit once ULP distance fcn is written

@private
@param {number} a - first number
@param {number} b - second number
@returns {integer} index