Mysql: add Encode impl for &[T]

[dyc3]

Is your feature request related to a problem? Please describe.
I was looking for an easy way to do bulk SELECTs by passing in an array of values (in my case, integer primary keys). I came up with this query:

SELECT id, username, first_name, last_name, profile_photo_url
        FROM UserProfiles
        WHERE id IN (?);

I went on to find that the Encode impl for &[T] was only present for Postgres

Describe the solution you'd like

Basically, I want impl<'q, T> Encode<'q, Mysql> for &[T] where T: Encode<'q, Mysql>

Describe alternatives you've considered
The alternative is to just make single queries in a loop. Another alternative is to build a dynamic query where I append OR id == ? to the where clause for every id I want to grab.

Additional context
None

[abonander]

Those impls exist for Postgres because it supports arrays as a native datatype. MySQL does not.

To support binding an array in that case, we'd have to implement some sort of comma-expansion which is a planned feature covered by #875.