Support for primitive-uses backend

[ayakayorihiro]

This is a small PR adding support for the primitive-uses backend that lists all of the unique usages of primitives in a Calyx program!

Once I merge the recent Calyx PR, the primitive-uses backend can be used by running fud2 with --to primitive-uses-json.

The next PR will consist of having fud2 automate the below process:

1. generating FIRRTL via the firrtl backend
2. getting info about primitive uses via the primitive-uses backend
3. running a Python script (currently sitting in the generate-firrtl-mods branch in the Calyx repo) that generates FIRRTL primitive code
4. putting the FIRRTL from (1) and (3) together

[sampsyo]

Excellent; looks great!!