Works around Coq bug #7731

The notation was parsing-only and all it did was reorder the arguments for
from_option.  This creates just a needless divergence between what is written
and what is printed.  Also, removing it frees the name for maybe introducing a
function or notation `default` with a type like `T -> option T -> T`.

This fixes issue #12.

This followed from discussions in https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/134

Terminology taken from "A Fresh Look at Separation Algebras and Share"
by Dockins et al.