Properly generate new names for sockets/files

[co-dan]

fileName https://github.com/co-dan/interactive-diagrams/blob/f69750f7c8c2fcea85f3a166c917c07481503492/eval-api/src/Eval/EvalSettings.hs#L39 is essentially useless, since we are running everything in ReaderT. Filename is used here: https://github.com/co-dan/interactive-diagrams/blob/f69750f7c8c2fcea85f3a166c917c07481503492/eval-api/src/Eval.hs#L149

There are several options:

• Put the name generation function in the library, generate a name for a socket in place
• Make API functions accept another parameter - filepath
• Switch to StateT so that fileName (and other fields of EvalSettings) can be modified

[co-dan]

If the fixed number of workers architecture (as mentioned in https://github.com/co-dan/interactive-diagrams/issues/6) is adopted, it is going to be possible to generate a unique name for each worker during the initialization. Therefore, switching to that strategy would solve two problems at once.

[co-dan]

Not longer relevant See https://github.com/co-dan/interactive-diagrams/issues/6