[REG 1.0.1] match no longer returns an lvalue

[pbackus]

@safe unittest {
	SumType!int x = 123;

	x.match!(ref (ref int n) => n) = 456;
	// Error: match(x) is not an lvalue and cannot be modified

	assert(x.match!((int n) => n == 456));
}

This regression was introduced by PR #50, which inadvertently removed the ref attribute from matchImpl.