Support more than 64 fields in a struct

[samuel]

Currently a uint64 is used to track required fields. This limits a struct to fields with ID < 64. Ideally this limit should be removed.

[alecthomas]

Is this still an issue?

[samuel]

Unfortunately so: https://github.com/samuel/go-thrift/blob/master/thrift/thrift.go#L204

[lc0]

@alecthomas thanks for your PR, for now, will use your branch.
@samuel could we please merge it? Sounds like a reasonable change.