Make parser combiner lazy

[RCmerci]

related to #209

[SGrondin]

I've been running into stack overflow on 0.15.0 as well, which is surprising to me because #186 and #187 are very similar to the changes I've made on my old fork which does not have stack overflow issues.

It turns out the reason is the changes made by #186 and #187 aren't fully lazy.

This PR fixed all remaining stack overflows on 0.15.0 for me! 🎉 It passes all my tests on SZXX. However the impact on performance seems to be a slowdown of ~55%. I'd still support merging it, maybe along with #217 to (more than) cancel out the impact on performance?

[edwintorok]

Lazy is not thread safe and can sometimes segfault on OCaml 4, see mirage/ocaml-uri#178.
I'm about to submit a PR that removes the existing Lazy from the code.
Please use a ref instead (or ensure that the lazy value is not observable from a concurrent/parallel context by immediately forcing it)