rust: add snippets for crate, pub fns, and constructor

[balajisivaraman]

Some useful snippets for Rust mode that I've found wanting.

• The extern crate and the new ones are hopefully straightforward. (These are equivalent to the ones I found useful in Vim Snippets.)
• The pub variants of all the function snippets is something I've wanted for a long time. Right now I sometimes forget to add pub in front and have to go back to the beginning line. Other times, I'll usually add pub and then begin the snippet. With the newly added snippets, I hope to provide a canonical way to do them that are analogous to the non-pub counterparts.

[AndreaCrotti]

It's a bit of a shame that these snippets are exactly the same as existing snippet declarations with just a prefixed pub.
Apparently it's possible to have "subsnippets" but the stacked expansion is kind of disabled by default

yas-triggers-in-field is a variable defined in ‘yasnippet.el’.
Its value is nil

Documentation:
If non-nil, allow stacked expansions (snippets inside snippets).

So maybe it's not worth trying to be too smart.
Another option would be to always add pub in front allowing to remove it easily if not needed, which can be done just by modifying the existing snippets like

${1:pub }fn ....

@balajisivaraman would that be ok? How much is the split roughly between public and not public functions?

[balajisivaraman]

@AndreaCrotti, Thank you for having a look at this. Sorry for not responding earlier, but I'm currently out and unable to address your concerns immediately. I should be back in a week. I will have a look at what is the split between pub and non-pub functions in some Rust codebases I work with and update the PR accordingly.

[balajisivaraman]

@AndreaCrotti, I've checked in a sample of the major repos I regularly use (riggrep, rust-lang, crater etc.), and the lean seems to be more towards non-public functions. I would still prefer to have the pub variants, since the Vim snippets module also has them. However, if you feel strongly about this, I wouldn't mind removing them and updating the PR to only have the extern crate snippet for the time being.

[AndreaCrotti]

Yes ok I think it's fine, sounds sufficiently mnemonic to me as well to prefix with a p.

[balajisivaraman]

@AndreaCrotti, Thank you so much! 👍