Add ReferenceSequence display summaries

[jeromekelleher]

#1944 added a basic repr for the ReferenceSequence class, but no str or repr_html implementations.

We want these to show useful information like the length of the reference sequence, and maybe the first as last 10 chars.

We should also incorporate this into the TreeSequence and TableCollection summaries.

Add tests to the test_reference_sequence.py file near the repr tests.

[benjeffery]

Closing for inactivity and labelling "future", please re-open if you plan to work on this.