Update to hashbrown 0.12

[alice-i-cecile]

Objective

• A new version of the hashbrown crate that we use for HashSet and HashMap is available
• This has some nice new toys, like get_many_mut

Solution

• Bump our version number

Context

Here's hashbrown's changelog.

[mockersf]

#4004 (comment)

Dependant is enabled, if a dependency isn't updated there's usually a blocker

[alice-i-cecile]

Duplicate of #4004 apparently.

I'd like to note that I didn't opt for 0.12 in #3963 because our dependencies still use 0.11. We should hold off on upgrading until our dependencies catch up.

We should see what the duplicate dependencies checks show...