convert SegmentReader into a Trait

[PSeitz]

Motivation

Abstract tantivy away from its storage format, by having SegmentReader as trait, which provides access to things like inverted index, columnar storage and doc store. This will allow to use tantivy with a different storage format.

Changes

This a first minimal PR:

• convert SegmentReader into a Trait
• rename old SegmentReader to TantivySegmentReader
• remove get_fieldnorms_reader and use field_norm_readers instead.

SegmentReader has typically few function calls, so we should be able use dyn SegmentReader everywhere instead of generics.

Follow-up

Some structs returned by SegmentReader are still bound to the storage format, e.g. InvertedIndexReader