Make library usable on embedded devices

[teythoon]

I'd like to use lesspass on an embedded device, that means making it no_std and preferably also avoiding allocations and dependencies using allocations. I have some patches towards this, but wanted to discuss #4 and the overall goal first.

[71]

I think all allocations performed within the library itself can be avoided since we can compute the length of each vector ahead of time (and in some cases determine a constant upper bound). The problem is with the num_bigint dependency, which uses Vec behind the scenes and therefore allocates.

That bigint also has a constant maximum size (32 bytes), but I'm not aware of any no_std bigint library. Maybe we could provide our own implementation of these functions in no_std, with a potential loss of efficiency in that case?

[71]

AFAIK, 21cfc80 makes the library fully no_std compatible. There are no no_std specific tests, though. Feel free to add some if you want (especially for corner cases, like having long entropy buffers).