Add const generic to `Hrp` to reduce its size

[nazar-pc]

Hrp right now stores an array with 83 bytes and a size on top of that. On 64-bit platform that is 91 byte to carry around, which is unfortunate since in most cases human-readable part is just a few characters.

The size can be changed to just u8 and be sufficient, but I think the bigger problem is max allocation for the buffer upfront. It'd be nice to try and make the size of the buffer const generic, but I'm not yet sure how wide the implications for that will be in terms of API changes.

Originally suggested in #216 (comment)

[tcharding]

Thanks for the issue. As this describes an request for optimization do you have some usecase that is suffering from the size of the Hrp type? Otherwise I would think that any reduction in ergonomics of the API is not warranted. Early optimization being the root of all evil and all that.

[nazar-pc]

I actually don't have a huge need for this, just felt really wasteful using that much memory for typical 2-3 bytes of actual content and decided to report it.
Especially in cases where it is returned as a value from parsing function, where RVO will probably not happen.

If there was a code that deals with encoding/decoding a lot, this might be non-negligible, but in most case that is certainly not the case.

[tcharding]

Cool, thanks man.