Sync `x/gov` fork to the v0.50 version

[giunatale]

We need to make several changes to the x/gov module present in this repo to align it with the updated v0.50 version. An example being the use of collections.

This is a prerequisite for example for #73 so that the governors feature can make use of collections for the added data structures

[julienrbrt]

I think this is the perfect occasion to do #181 (comment), instead of resyncing it.

[giunatale]

sounds good!