check-docstrings and imports

[yav]

At the moment we don't run doc-strings on imported modules, but perhaps we should? Here's an example:

/**
```
"B"
```
*/
module B  where
  /* empty */

module A where
  import B

Running check-docstrings after loading A doesn't do anything.

Cryptol> :l A.cry 
Loading module Cryptol
Loading module B
Loading module A
A> :check-docstrings 

Checking A

Successes: 0, No fences: 1, Failures: 0

[glguy]

I think it only makes sense to recurse the import tree in the trivial examples. Often we'd have modules being defined because they are being used in multiple places. We could avoid rechecking this modules multiple times with caching, but that seems like working around bad design with a speed-hack. For a project with multiple top-level leaf modules you'd already need to manually enumerate what those top-level modules are. It seems more principled to have a list of the modules you that want to check which includes those that might be imported by other modules.