Replace recursion of Repeat::parse_helper with a loop

[zhengyao-lin]

I noticed that the original parse_help still gets compiled to a recursive function. So I replaced it with a loop instead. No particular performance impact though.

[y1ca1]

1. Does it make sense to remove the parse_helper entirely?
2. For consistency, do we want serialize to be a loop as well?

[zhengyao-lin]

1. Yeah probably;
2. Makes sense; will get to that at some point!