Tree schema that supports shared and unique

[oflatt]

This PR adds a tree schema that can support both shared terms and unique terms.
Compared to tree_unique:

• Adds a new variant of Id called Shared, which can be used for shared terms
• Adds a test in the id analysis to prevent mixing shared and unique ids in terms
• Adds ids to branches of switch statements, allowing for path-condition based optimizations. Switch statements must now have Branches as children. Adds a test that ensures this.
• Deletes most optimizations, but keeps most utility
• Adds IsValidShared and IsValidUnique, subsets of IsValid for shared and not-shared optimizations.
  IsValid = IsValidShared \union IsValidUnique

[rtjoa]

Looks good (just skimmed).

Note: Before people start hacking on their optimizations, I think we should do follow-ups on this that implement typed args and lets/loops-are-lambdas.

Nit: I think the directory should be called tree rather than tree_shared. Then, once all optimizations are ported, we'll delete tree_unique_args.