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.
A logic issue existed in the Pipe, as it was mistakenly relying on
`IsolatedRoot` instead of the isolated function. That was breaking
things if those functions were to be removed. This issue has been
addressed by correcting the behaviour of the Pipe.
Alessandro Di Federico