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 #2577
coq
stdpp-coq8.5

00:02:52

passed #2578
coq
stdpp-coq8.6

00:02:26