Shoobx-immutable

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- Nothing changed yet.

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- Introduced `__im_version__` to `IRevisionedImmutable` and use it instead of
timestamps to create a chronological order of revisions. (Timestamps might be
slightly different accross servers and cause bad history.)

- Do not duplicate implementation of `__im_update__()` in
`RevisionedImmutableBase`. Use `__im_[before|after]_update__()` to do all
revision-related tasks.

- Tweak `copy()` implementation for `ImmutableList` and `ImmutableDict`.

- Properly implement `ImmutableDict.fromkeys()`.

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- Fix `ImmutableList.copy()` to just work when locked. This allows for only
making a shallow clone, since any update will cause a deep copy and thus
immutability is guaranteed.

- Implemented `ImmutableDict.copy()`. Raise error on `ImmutableDict.fromkeys()`.

- `ImmutableContainer` also needs an updated `_pj_column_fields` list.

- Minor test fixes.

- Minor documentation fixes and code comment enhancements.

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- Add API documentation.

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- Moved documentation to Read the Docs.

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- Add some readable documentation.

- Added high-level `shoobx.immutable.update(im, *args, **kw)` function.

- Implemented `__repr__()` for `ImmutableSet` to mimic behavior of
`ImmutableDict` and `ImmutableList`.