Merge pull request #18 from savi-lang/remove/until-methods

Remove `*_until`-style iteration methods.

Merge pull request #17 from savi-lang/add/get-or-insert

Add `get_or_insert` method.

Merge pull request #16 from savi-lang/add/is-empty

Add `Map.Readable.is_empty` and `.is_not_empty` convenience methods.

Merge pull request #15 from savi-lang/update/latest-savi

Update for latest Savi syntax.

Add `Map` library code.