Deduplication of alts does not preserve capture groups

[hvdijk]

$ libfsm/build/bin/re -p -rpcre -lpcre '(foo)|(foo)'
(foo)
$ libfsm/build/bin/re -p -rpcre -lpcre '(foo)|(bar)'
(bar)|(foo)

In general, deduplicating alts is a good simple optimisation to speed up the generation of an FSM, but for regexes containing capture groups, the optimisation is not valid.

[katef]

There's also reordering, and I suppose we either refrain from doing that, or we error about it as an unsupported situation.

[silentbicycle]

This has definitely been addressed in my capture branch.