Revise string interfaces for dictionary elements

[bocchino]

Closes #407. See that issue for the rationale.

Notes:

• This PR incorporates changes from Fix C++ code gen for string records #408. Merge that one before reviewing.
• This PR will require some minor changes to user test code. For example, you can no longer pass a bare char* to the sendCmd interface; you have to wrap your char* in an F Prime string constructor. However, I think we want to go in this direction. We should avoid implicit conversions, e.g., implicitly converting char* to an F Prime string type. Eventually, if possible, we should make single-argument constructors explicit.