Fix clippy lint clippy::get-first

[vbrandl]

A new clippy lint fails on slice.get(0) and proposes to use slice.first() instead. Here is an example of a failing build: https://github.com/vbrandl/hoc/runs/7832070989?check_suite_focus=true

And here is the lint in question: https://rust-lang.github.io/rust-clippy/master/index.html#get_first

[kaj]

Seems pretty clear. Thanks for the contribution!

[vbrandl]

Could you also publish a new release to fix the lint in projects that consume the library?

[kaj]

Yes, there's something other fix that should get released as well. I wanted to make a release of rsass first and include an update of that, but can't seem to get that done. Will try to cut a release of at least ructe this weekend.