Consider using darling for proc macro parsing

[greyblake]

See: https://github.com/TedDriggs/darling

It's used in many projects already and if it's flexible enough, it can simplify proc macro parsing a lot.

The list of similar libs for a reference:

• darling
• deluxe
• syner

[greyblake]

I gave an attempt to darling. The result can be seen at the moment in branch v031-experiment-darling
The library is very cool and make some things simpler.
However, in case of nutype, I had to face number of small issues that required workarounds.
There were some tiny things, (e.g. inability to handle i128/u128 which was easily addressed with a tiny PR.

There were a bit weird things like Expr does not parse str lit correctly: TedDriggs/darling#229

There were bigger things, like inability to parse negative integer.

After all it feels like the effort is not worth it. There are too much work around and too many intermediate types.
So I've decided to just parse things with syn::parse. It results into a way more straight forward code and should be at least a few times faster.