Replace lazy_static with phf and once_cell

[GnomedDev]

• Reduces macro magic in the case of replacing with once_cell::sync::Lazy and removes a lot of lines due to no indentation required.
• Improves runtime performance in the case of replacing lazy_static'd HashMap with phf::Map by creating the maps at compile time, avoiding even one allocation at runtime

[pdeljanov]

Hi @GnomedDev,

It looks like you put quite a lot of work into this. Unfortunately, I see some problems that give me pause:

1. Regarding the replacement of lazy_static with once_cell. According to the once_cell docs, lazy_static has an advantage in terms of supporting #![no_std]. I think we do want to eventually support no_std and we having a tracking ticket here #![no_std] support #88. Are there any other benefits besides code brevity that outweigh this drawback?
2. In terms phf there are two issues I see:
   • First, phf requires a MSRV of 1.60, while we support down to 1.53. Rust 1.60 is rather new. I was thinking of moving to 1.56 next. Either way, increasing the MSRV would be done for v0.6.
   • Second, since the hash tables are being generated at compile time, how does that affect binary size vs. the existing solution? We've already had a few issues raised regarding binary size and I'd prefer trading a few CPU cycles for a smaller binary.

[GnomedDev]

Hey, sorry for completely missing your response.

1. once_cell is now a part of the Rust standard library, so the library should probably move to that if a breaking change is in order
2. I personally think if this change would be made, it move to a simple match statement instead of dragging in a dependency, if possible.

Since this has been left stale and is now conflicting so much, I will be closing this PR, feel free to use the ideas or code for a future PR.