clippy: Replaces .get(0) with .first()

[brooksprumo]

Problem

There are nightly clippy warnings that will prevent upgrading our stable Rust version.

For this PR, it's this one: https://rust-lang.github.io/rust-clippy/master/index.html#/get(0)

Summary of Changes

Replaces .get(0) with .first().

[brooksprumo]

One could argue that the original .get(0) is better since we do get() on 0, 1, then 2. We'd need to add a clippy-allow to go back to that way.

Using .first() doesn't bother me here; I have a slight preference for .first() versus adding a clippy-allow exception.

[t-nelson]

i could argue that using the get() interface here is a bit dubious in the first place. same with anywhere else we're doing a .get(n).unwrap()

[codecov]

Codecov Report

Merging #34048 (b509d61) into master (47a98eb) will decrease coverage by 0.1%.
Report is 3 commits behind head on master.
The diff coverage is 91.6%.

@@            Coverage Diff            @@
##           master   #34048     +/-   ##
=========================================
- Coverage    81.9%    81.9%   -0.1%     
=========================================
  Files         811      811             
  Lines      219552   219552             
=========================================
- Hits       179902   179858     -44     
- Misses      39650    39694     +44     

[t-nelson]

lgtm

[t-nelson]

i could argue that using the get() interface here is a bit dubious in the first place. same with anywhere else we're doing a .get(n).unwrap()