Generalize `FunctionStringMap` into `detail::GenericStringMap`, which
supports any `Depth=1` rank. `FunctionStringMap` remains as an `using`
with `Rank = ranks::Function`.
Replace the stub implementation of invalidation with the proper
implementation. A ReadPathCache is added to each global so that it can
keep tracks of what target are associated to which read paths.
Change the serialization and deserialzation functions of
`FunctionStringMap` to use the GzipTarFile. Also change che filename
of the underlying containers accordingly.
Alessandro Di Federico