tail recursive key parsing

[ygrek]

Continuing the work in #78
Now it is possible to import the following key ([email protected]) with stack lowered to 1MB (ulimit -s 1024) :

1 selfsigs, 5 uids (101025 packets), 1 subkeys (273 packets)

TODO:

• extract the anomaly key and make a test case
• measure speed

[wiml]

I wonder if it would make sense to use a delayed/lazy sequence for some of this (the Seq module in stdlib)?

[ygrek]

my only concern is List.rev on signatures, but i don't expect it to be costly on average. I don't want to raise minimum ocaml version or add more dependencies until making a release.