Support partial assignment of Logic

[mkorbel1]

Motivation

There is now support for assignSubset of a LogicArray, which was a natural convenience automation. The idea of partial assignment for a Logic has been long-requested, but conceptually did not work with the modelling for signals in ROHD. However, with LogicArray support, there's a new opportunity to cleanly support it with a little automation.

Desired solution

Add a function assignSubset to Logic which creates a new bit-blasted LogicArray to drive that original Logic, then calls assignSubset on that new driver array. This gives a clean API and hides the dirty management of blasted drivers from the user.

Alternatives considered

Let users figure out their own automation via swizzle and or LogicArray as necessary.

Additional details

Related to #378, but separate

This approach is less efficient for multi-bit drivers since it blasts the whole intermediate into bits. A swizzle approach is more efficient in terms of simulation and will create nicer generated outputs. However, an API definition for varying width subset drivers, as well as a robust implementation, would be substantially more complicated. It would also require a "resolution" time for when to finally compress the signals into one swizzle, which doesn't play well with the rest of the connectivity APIs. Perhaps this could be revisited if #97 is ever supported.