Switch `std::string` -> hashes for label resolution. (#2215)

Use the simple but well-known FNV-1a hash function to map
`cell_tag_type` aka `std::string` to an `uint64_t`
for label resolution. The former type has a size of 32B or more and the
latter 8B, thus cutting the storage and bandwidth
requirements by 3/4. 

The hash function is implemented from the reference given on the
authors' website/wikipedia and is extremely
simple. If we ever experience issues, we might consider switching this
to something of higher quality via an
external library, candidates are `xxHASH` and `Murmur3`.

https://github.com/Cyan4973/xxHash

Note: This should further relieve the memory pressure on larger scale
simulation as formulated in #1969 and make
#2005 less urgent.

There is no performance impact (at laptop scale), but the memory savings
are worth it.