Delete im

[graydon]

This still has some TODOs in the form of error codes to consolidate/reassign, but the code is otherwise I think as it should be.

This PR deletes the im library previously backing the Host collection objects (maps and vecs), and also reworks the comparison system to avoid #579. It replaces both vecs and maps with simple eagerly-copied std::Vecs (maps are sorted std::Vec<(K,V)>s). It also eliminates all the WeakHost and EnvVal usage in the Host; with a little more work plus catching up the SDK it should be possible to remove EnvVal entirely. In addition to removing WeakHost it also removes all the auxiliary pointers to Budget stored in "metered" host objects; a single Budget and/or Host is passed in to each operation that needs it.

The only odd part of this is that maps and vecs no longer "share substructure" via the BTree/RRB code in im -- every edit makes a full copy -- but some consideration suggests this probably isn't a huge issue:

• The host vecs and maps still "share substructure" of any host objects they refer to; they're just slabs of RawVals, so are still "shallow" in that sense (and the storage map entries are changed from Box to Rc, so cloning them is "shallow" now too).
• For small maps (less than 64 entries) im would be holding everything in a single BTree or RRB node anyways, which means this is no less efficient. The sharing only kicked in on larger structures, and ... we're likely not going to have big maps or vecs available to users.
• Since we're removing EnvVal (which used to comprise the map and vec entries) the data consumption actually shrinks with this change (again so long as you're dealing with small maps or vecs).
• I've been fairly careful to make all the operations that build new maps or vecs do so with a single exact-size allocation, again making these smaller for small maps and also relatively efficient. We can make them more efficient still by going to a per-host arena allocator in the future, if we want.

Comments welcome!

[leighmcculloch]

Is it happening?

[graydon]

@leighmcculloch I believe so, if you would be inclined to review! I have pushed a rebase and it should be ready now.

(It'd be good to do soonish since I will be out after thursday, and this will bitrot fairly quickly)

[jayz22]

Half way through my review, will continue tomorrow.

[leighmcculloch]

👍🏻. My knowledge of some of the intricacies is limited now so I think this should probably get an ✅ from another person, but it looks good to me overall.