TryInto trait implementation

[nazar-pc]

::n() method is useful, but it would be even nicer to have standard implementation for TryInto trait in addition or instead of it.

[dtolnay]

I would prefer to stick with just the associated function for now, with the TryFrom impl being handwritten if the author wants one. This crate is <100 lines though, so it would be easy to fork into a library that produces TryFrom.

There's also some talk of putting a TryFrom derive in the standard library for this use case: rust-lang/rfcs#2783.

[nazar-pc]

Subscribed to that issue. I just wanted to have a C-like enum that can deref to an integer and same way to use .try_into() for going backwards.

Boilerplate I had before was quote long and this library was the closest thing I found except it is missing actual deref and TryInto/TryFrom support, so code is still not as nice as it could have been.