Add unic-segment component

[behnam]

Make all unic-ucd-segment properties Total, as turns out using these properties is easier as a total property, instead of partial, as we implemented initially. The reason for it is the fact that the default value (called Other in this context) is usually used in the same level as other values.

The algorithm implementation is based on unicode-segmentation, with changes to use unic-ucd-segment.

The API is kept minimal at the moment, with only segment boundry objects and their indices objects. As we expand our string-level APIs, we can add on what's needed.

The tests for the algorithm are imported. Also new test is created for unic-ucd-segment, using the same source.

Closes https://github.com/behnam/rust-unic/issues/135

[behnam]

bors: r+

[bors[bot]]

Build succeeded

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