shelve: drop pickle usage
Pickle was used to the `shelvedstate` file. However the content of the file is
very simple and we can handle serialisation ourself. Not using pickle is a net
win.
Note incrementing the format version as no releases have been done so far.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a