Remove redundant Tuple.opEquals

Today struct type equality `a == b` is expanded to `a.tupleof == b.tupleof`, so opEquals method is unnecessary for `std.tyoecons.Tuple`.
Make VariantN.toHash() const nothrow @safe

Assume that the call `fptr(OpID.getTypeInfo, ..)` in `@property VariantN.type()` is logically nothrow.