Replace boost::container::flat_map with absl::btree_map in P4Tools.

[fruffy]

Yet another variant of #4667 and #4713 but a bit more conservative.

btree_map is a memory-efficient variant of std::map which does not require hashing. We can use it to replace boost::container::flat_map in P4Tools.

Reason why I am splitting this PR out:

#4667 introduces some hash functions which are still too inefficient and needs improvement.

#4713 introduces a new data structure (flat_map) which should be reviewed separately.

The other two PRs will be rebased after this one is in.

[asl]

Note that btree_map has different iterator invalidation rules as compared to std::map. Not sure if it matters, but worth double checking.

[fruffy]

Note that btree_map has different iterator invalidation rules as compared to std::map. Not sure if it matters, but worth double checking.

We do not remove elements from these maps (although we should...) and I believe we only iterate for debugging. Pointer stability could be a problem but can't currently think of a case where.

[vlstill]

Conceptually looks good to me, just some code quality suggestions.

[fruffy]

I noticed solver.h depends on IR::SymbolicVariable, which is only defined in the p4tools back end. Either we move the definitions of these files to the top-level def files, or move the solver class back to the p4tools code.