Chunked seekable stream reader

[halgari]

resolves #565

This implements a system for lazy loading of data via chunked streams, main classes implemented in this PR:

• ChunkedStream - takes a IChunkedStreamSource for loading chunks with a size and count specified by the source. ChunkedStream is a seekable, read-only stream that supports both async and sync read methods. Chunks are stored in a LRU cache that defaults to a size of 16. This should allow for random access to be rather fast, as long as the current working set of data fits within the chunks that fit in the LRU cache
• IChunkedStreamSource - a interface for defining a chunked stream, has both sync and async GetChunk methods so that each type of threading model remains performant
• LightweightLRUCache - a very lightweight LRU cache that does not support threading in any manner (we don't need it here), but because of that can be a struct embedded in ChunkedStream.

[Sewer56]

All's good aside from the small notes.

[Sewer56]

This can use DangerousGetReferenceAt to eliminate bounds checks.

[Sewer56]

This is fine for merging in current state.

Do consider eliminating some additional bounds checks with DangerousGetReferenceAt where you feel applicable; as it would shave some instructions here and there. That said, all's good now.