Add a general purpose readonly interace to BitSet [LUCENE-6805]

BitSet has many more readonly methods compared to Bits. Similarly, BitSet has many more write methods compared to MutableBits.

This Jira issue is to add a new ImmutableBits interface to BitSet that includes all ready only methods of BitSet.

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Migrated from [LUCENE-6805](https://issues.apache.org/jira/browse/LUCENE-6805) by Selva Kumar, updated Sep 27 2015

Start by locating the BitSet, Bits, and MutableBits definitions and compare the read-only methods exposed by BitSet with the existing interfaces. Review the linked LUCENE-6805 discussion for the intended API scope. Done means BitSet has a general-purpose ImmutableBits interface containing all of its read-only methods.

Written by the indexing model from the issue text.