introduce [default] as abbreviation for [from_option id], and use it

See merge request robbertkrebbers/coq-stdpp!32

Remove the `default` notation for options

See merge request robbertkrebbers/coq-stdpp!31

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`.

Add `seq_set_pred_disjoint`

See merge request robbertkrebbers/coq-stdpp!29

This fixes issue #12.

Notations for relations with explicit type arguments

See merge request robbertkrebbers/coq-stdpp!30

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.