* BlockByteRange -> ByteRange
* RangeFetcher.FetchBlocks -> RangeFetcher.FetchRanges

This commit:
* Replaces the body of FetchBlocks with two main functions,
  fillBlocksFromMultipartReader and fillBlocksFromContiguousReader.
  - "Multipart" uses "Contiguous" to do its heavy lifting.
  - "Contiguous" reads a number of blocks from a single stream and returns
  how many were successfully filled. Blocks are filled based on order and
  length alone.
* Splits out FetchBlocks' response validator.

This is a platform on which to support range coalescing. Previously,
HTTPRanger couldn't do range coalescing because there was a 1:1 mapping
between multipart parts and blocks returned.

Closes https://github.com/DHowett/ranger/issues/9.
Closes https://gitlab.howett.net/DHowett/ranger/issues/8.

Closes https://github.com/DHowett/ranger/pull/3.
Closes https://gitlab.howett.net/DHowett/ranger/merge_requests/3

.

Initially-authored-by: Jeremy Archer <open-source@fatlotus.com>

Refs https://github.com/DHowett/ranger/issues/8.
Refs https://gitlab.howett.net/DHowett/ranger/issues/7.

Refs https://github.com/DHowett/ranger/issues/8.
Refs https://gitlab.howett.net/DHowett/ranger/issues/7.

Closes https://github.com/DHowett/ranger/pull/4.
Closes https://gitlab.howett.net/DHowett/ranger/merge_requests/4.

Closes https://github.com/DHowett/ranger/issues/5.
Closes https://gitlab.howett.net/DHowett/ranger/issues/5.