Feature/alloc feature

[Pzixel]

Closes #999

It just adds feature gate everywhere and showcases one example how some string usages can be replaced with arrayvec. It's a bit unfortunate in a sense that sequences with N > 10**17 elements will have a slightly worse error message, but otherwise it remove an allocation in here for free and works in no_std no alloc scenarios.

The subsequent PRs can be used to remove alloc requirements for the code (e.g. mod utils doesn't seem to need that many allocs).