Simplify ResourceMaps

[nical]

Connections

This PR is rebased on top of #4878
Another step towards #4787

Description

ResourceMaps had a rather convoluted system for erasing types that isn't needed anywhere except in one place in the triage_resource function. Everywhere else we are always dealing with specific types so using a member of the resource maps is simpler than going through an abstraction. More importantly there was a constraint that all contents of the resource maps implement the Resource trait which got in the way of some of the ongoing buffer snatching changes.

This commit simplifies this by removing the abstraction. Each resource type has its hash map directly in ResourceMaps and it is easier to have different requirements and behaviors depending on the type of the each resource.

The main wrinkle is that for triage_resource to still work without knowing about the resource type, it takes a callback to go from ResourceMaps to the right hash map. it could be done with some trickery in ResourceMap but I found that this was the simplest way albeit not the most elegant.

Checklist

• Run cargo fmt.
• Run cargo clippy.
• Run cargo xtask test to run tests.

[nical]

After some more poking, I think I won't actually need this to move forward with buffer snatching (but I prefer the way it works after this change so I'm keen on landing it).

[Wumpf]

nice, makes sense to use the fact that we always know the type!
only cosmetic comments. I think the callback on triage_resource isn't all that bad indeed, seems like the least invasive thing for the moment :)