copilot-c99: Allow multiple triggers with the same name

[fdedden]

Extends the C99 backend to allow multiple references to the same trigger:

spec = do
  trigger "f" guard0 [arg $ constI8 10]
  trigger "f" guard1 [arg $ constI8 12]

Note that the guards may overlap, which will mean the trigger is called twice in that iteration.

As C99 does not allow multiple function definitions sharing the same name, we can only allow a single type per trigger function. The following is explicitly disallowed by the backend:

spec = do
  trigger "f" guard0 [arg $ constI8 10]
  trigger "f" guard1 [arg $ constI8 12, arg $ constI8 15]

Upon compilation, the user gets an error message:

Copilot error: attempt at compiling specification with conflicting trigger definitions.
Multiple triggers have the same name, but different argument types.

Feel free to make any modification necessary to my commits.