Add Display and DisplayAlternative wrapper types

[tyranron]

Follows #16, #17

Motivation

Applying the base62::encode_fmt() in my codebases, I've found that it doesn't play smooth with macro-based or non-transparent APIs (like derive_more::Display or thiserror::Error).

Consider the code before applying #17:

#[derive(derive_more::Display)]
#[display("{:0>22}", base62::encode(*_0))]
pub struct Version(u128);

This code now should be written "by hand", since base62::encode() cannot be substituted with base62::encode_fmt() trivially here.

Solution

This PR introduces two functions: base62::fmt() and base62::fmt_alt(). They allow applying base62 encoding easily in any format!()-like expressions, still avoiding unnecessary allocations.

#[derive(derive_more::Display)]
#[display("{:0>22}", base62::fmt(*_0))]
pub struct Version(u128);

[tyranron]

Alternative naming could be: base62::Fmt and base62::FmtAlternative.

[fbernier]

Hey, thanks for the great explanation. I'm not against adding this. I left a few comments so let me know what you think and if you want to do these changes yourself.

Thanks!

[tyranron]

@fbernier reworked!

[fbernier]

Much cleaner, thanks! Cutting a release soon.

[fbernier]

I ended up reworking things a bit. I replaced encode_fmt and encode_fmt_alternative with the fmt helper functions. One can use format! or write! macro on these functions as needed.

Released as 2.2.0

Thanks again and let me know if I missed anything