Convtools

- fixed `c.this.datetime_parse` with default when uncovered data remains

- sped up `c.item(..., default=...)` for python 3.10+ by a c-extension

- added experimental `c.this.window(...).over(...)` and `c.WindowFuncs`
- added experimental support of `key` as conversion or tuple of conversions to
`c.sort` and `c.this.sort`
* `c.this.asc(none_last=None, none_first=None)`
* `c.this.desc(none_last=None, none_first=None)`
* `c.sorting_key(*keys)` standalone helper to generate callables for
`sorted`'s `key` parameter
- improved `Table.from_rows` not to raise StopIteration on empty sequences when
no header inference is required

- fixed `Table.join` for dict-based rows after `rename` usage (_no breaking
changes as it used to break completely_)

- added experimental `of_` parameter to `c.Mut.set_item`, `c.Mut.set_attr`,
`c.Mut.del_item`, `c.Mut.del_attr` to mutate non-root objects

- added experimental `Table.pivot` method