Adds FromStr and TryFrom<&str> traits to enums implementations

[xanathar]

This adds the generation of FromStr and TryFrom<&str> trait impls for enums.

• Contrary to FromGlib / IntoGlib, I removed the doc(hidden) attribute so that the traits get documented by cargo doc
• As discussed on matrix, the error type is BoolError
• The parsing is based on the enum member's nick; for members with no nick specified, the original "name" is used

This does not use a configuration parameter. After implementing it, I'm leaning on the opinion that a configuration flag should be provided, as it's not entirely unlikely for somebody to have implemented FromStr or TryFrom in their /manual/ module, and if that implementation does not use the same nicks it can be a little painful to migrate.

Not part of this PR

These are not in the PR, might be desired in future PRs:

• Same support for bitfields (might conflict with bitfield's default parser)
• impl Display for $enum for the reverse operation

gtk-rs-core

Tested gtk-core-rs with this and it seems to work. Also a couple of personal projects build just fine.
If/when this gets merged, I can follow-up with a PR on gtk-rs-core.

[bilelmoussaoui]

Can you do a patch for gtk-rs-core/gtk4-rs already so we can see if the changes looks good and work?

[xanathar]

Of course, just give me a few days.