Implement std::convert::TryFrom<&str> for EnumString

[dspicher]

The EnumString macro provides std::str::FromStr.

I wonder whether it would be possible to also provide an implementation of the nearly equivalent std::convert::TryFrom<&str>?

[twz123]

Related: rust-lang/api-guidelines#182

[Peternator7]

This is probably doable. Our MSRV is 1.31 and TryFrom wasn't stablized until 1.34, but I believe there are some workarounds with cfg attrs to only include features for specific versions.

[dspicher]

Great, I will try to take a stab at this.

[dspicher]

The implementation is simple enough, I have coded up something here:
dspicher@d5e16d4

However, the requirement to make this work with Rust 1.31 (or rather 1.32 as is stated in the current strum README) is more difficult. cfg_version is still unstable. There is the rustversion crate one could use. However, this would force all users of EnumString to take this crate up in their dependencies once the macro is expanded. Or is there any way I don't know of to circumvent this? EDIT: Solved by inlining rustversion usage into the strum crate by annotating local methods. Thx @toxeus for the hint.