Rename multiset "union" into "disjoint union"

Closes #13

See merge request iris/stdpp!57

Also, use the union name/class/symbol for what's usually the union, and
define the intersection on multisets.

fix or silence Coq 8.10 warnings

See merge request !52

New `seq` operation on maps + consistency tweaks for `seq` operation on sets

See merge request !44

Consistently use `set_` prefix.

Closes #24

See merge request iris/stdpp!45

- Rename `gmap.to_gmap` into `gset_to_gmap`.
- Rename `gmap.of_gset` into `gset_to_propset`.
- Rename `coPset.to_Pset` into `coPset_to_Pset`.
- Rename `coPset.of_Pset` into `coPset_to_gset`.
- Rename `coPset.to_gset` into `coPset_to_gset`.
- Rename `coPset.of_gset` into `gset_to_coPset`.

The following `sed` script can be used for the first rename:

```
sed -i 's/to\_gmap/gset\_to\_gmap/g' $(find ./theories -name \*.v)
```

The latter is context sensitive, so was done manually.

Get rid of using `Collection` and favor `set` everywhere. Also, prefer conversion
functions that are called `X_to_Y`.

The following sed script performs most of the renaming, with the exception of:

- `set`, which has been renamed into `propset`. I couldn't do this rename
  using `sed` since it's too context sensitive.
- There was a spurious rename of `Vec.of_list`, which I correctly manually.
- Updating some section names and comments.

```
sed '
s/SimpleCollection/SemiSet/g;
s/FinCollection/FinSet/g;
s/CollectionMonad/MonadSet/g;
s/Collection/Set\_/g;
s/collection\_simple/set\_semi\_set/g;
s/fin\_collection/fin\_set/g;
s/collection\_monad\_simple/monad\_set\_semi\_set/g;
s/collection\_equiv/set\_equiv/g;
s/\bbset/boolset/g;
s/mkBSet/BoolSet/g;
s/mkSet/PropSet/g;
s/set\_equivalence/set\_equiv\_equivalence/g;
s/collection\_subseteq/set\_subseteq/g;
s/collection\_disjoint/set\_disjoint/g;
s/collection\_fold/set\_fold/g;
s/collection\_map/set\_map/g;
s/collection\_size/set\_size/g;
s/collection\_filter/set\_filter/g;
s/collection\_guard/set\_guard/g;
s/collection\_choose/set\_choose/g;
s/collection\_ind/set\_ind/g;
s/collection\_wf/set\_wf/g;
s/map\_to\_collection/map\_to\_set/g;
s/map\_of\_collection/set\_to\_map/g;
s/map\_of\_list/list\_to\_map/g;
s/map\_of\_to_list/list\_to\_map\_to\_list/g;
s/map\_to\_of\_list/map\_to\_list\_to\_map/g;
s/\bof\_list/list\_to\_set/g;
s/\bof\_option/option\_to\_set/g;
s/elem\_of\_of\_list/elem\_of\_list\_to\_set/g;
s/elem\_of\_of\_option/elem\_of\_option\_to\_set/g;
s/collection\_not\_subset\_inv/set\_not\_subset\_inv/g;
s/seq\_set/set\_seq/g;
s/collections/sets/g;
s/collection/set/g;
' -i $(find -name "*.v")
```

Confluent relations

See merge request !53

The name made no sense and it was not used anywhere to my knowledge. If it
was used anywhere, it would be very unreliable as it contained hints like
`rtc_trans` that would generally lead to loops.

Seal `fresh_generic`.

See merge request !54

Since `fresh_generic` is too inefficient for all practical purposes, we seal
off its definition. That way, Coq will not accidentally unfold it during
unification or other tactics.

This issue actually occurred in iGPS, as reported by Hai.