When working with `stdin` and `stdout`, the `addExtension` function
should return std::nullopt, since concatenating files is undesirable.
This fixes a bug where the output of the `StringMap` container would
have the index file appended to it, rendering an invalid tar.
Add a `Dirty` boolean to the `ContainerBase` class. This will be set to
true when the container mutates itself (e.g. remove, mergeBack, clear).
The dirty bit is reset when running `PipelineManager.store`.
Generalize `FunctionStringMap` into `detail::GenericStringMap`, which
supports any `Depth=1` rank. `FunctionStringMap` remains as an `using`
with `Rank = ranks::Function`.
Giacomo Vercesi