reduce memory usage of the construct_automata script

[lenawanel]

this is a series of commits aiming to increasing the speed and reduce the memory usage of the construct_automaton crate.
I don't see any further big gains except making Trigger hold &strs instead of strings, or parallelizing the loop at line 141.
making Triggers hold &str in particular might reduce the memory usage to an extend where it becomes possible to use the crate on the provided js grammar without an it being killed by OOM.
It would however be a breaking change, since the deserialized source all Automaton structs in fuzzers would need to live as long as them.

[andreafioraldi]

Hey, ty, have you extensively tested that the output is like before? In that case, LGTM we can merge it

[lenawanel]

I tested it with every provided grammar except the test1 grammar and up to a stack limit of 4 for the js grammar, and they produce the same output A before.