Rearrange libraries and executables to make purpose clearer

- Split `libcrawl` from cache implementations (new `libcrawl-caches`)
- Rename `libcrawld` to `libanansi-crawld`
- Rename `crawld` to `anansi-crawld`
- Replace `crawler-add` with a generic `anansi` tool of which `add` is one administrative function
