Make arguments for sig and sigT constructors and projections maximally implicit.

Some were already maximally implicit, some were not. Now it is consistent.
2 jobs for master in 5 minutes and 18 seconds
Status Job ID Name Coverage
  Test
passed stdpp-coq8.5 #2577
coq

00:02:52

passed stdpp-coq8.6 #2578
coq

00:02:26