Make cache-db.json into a binary file

[hdgarrood]

Summary

Replace cache-db.json with cache-db.cbor, using the same mechanism for serialization as we already do for externs files.

Motivation

The correctness of the incremental build system depends on an assumption that only the compiler is doing anything with cache-db files, so avoiding JSON is desirable because it also decreases the likelihood that other projects might start depending on them. It should help speed up serialization and deserialization a bit too.

[hdgarrood]

Alternatively we may want to just leave this file alone for now, and instead work towards cutoff in our incremental build system so that we can get rid of the cache-db.json file completely: #3996 (comment)

The cache-db.json file is only necessary because we don’t have cutoff. If we did, we could safely store the last seen timestamps and hashes of input files in their respective externs file (by not considering this information to be part of the module interface), and not need a separate cache-db.json file at all.