box_derive: New command to derive modules for boxes

[povik]

In certain uses cases you want to derive modules for blackbox instances, which are otherwise ignored by the hierarchy process. Here we add a new command, box_derive, by which one can ask to have modules derived for a selected set of blackbox instances.

I can think of two use cases:

• Functional modelling of blackboxes (the abc9 integration has an abc9_ops -prep_hier command to a similar end, we can merge those commands eventually)
• Deriving of one of the possible implementations for a blackbox (where the blackbox is a blackbox because an implementation hasn't been selected yet)

An incomplete test is attached.

[povik]

Help sample (updated after rename):

    box_derive [-base <base_module>] [-naming_attr <attr>] [selection]

As part of the assembly of the design hierarchy done by the 'hierarchy' command,
specializations of parametric modules are derived on demand: for each choice of
parameter values appearing in the design, a copy of the parametric module is
derived which is specialized to that choice.

This derivation process ignores blackboxes and whiteboxes (boxes). To supplement,
this 'box_derive' command can be used to request the derivation of modules based
on box instances appearing in the design, which is desirable in certain use
cases. Only the selected cells are considered as the instances that steer the
derivation process.

    -base <base_module>
        instead of deriving the module that directly corresponds to each box
        instance, derive a specialization of <base_module> (this option applies
        to all selected box cells)

    -naming_attr <attr>
        once a specialization is derived, use the value of the module attribute
        <attr> for a name which should be used for the derived module (this
        replaces the internal Yosys naming scheme in which the names of derived
        modules start with '$paramod$')

[povik]

Bikeshed: should this not be called just blackbox_derive? We have blackbox already, so it'd be inconsistent otherwise.

I don't have a strong preference, you could say this shouldn't be blackbox_derive because that gets to be too long.

[povik]

box_derive also an option

[whitequark]

box_derive seems nice, considering you also handle whiteboxes.