Add SortModules Transform (#1905)

* Add SortModules to transform to def-before-use

Adds a new transform, SortModules, that transforms a FIRRTL circuit
to enforce an invariant of modules and external modules being defined
before use. This transform is left as optional in the event that a
user may wish to have a quick way of getting the circuit to respect
this property as may be expected of some other tool, e.g., MLIR.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

* Add test of SortModules transform

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>