Support a file data iteration interface for large files

Summary: Ref T7149. A couple diffs down the line, this will let us emit chunked files without doing all the work up front or holding the entire file in RAM.

Test Plan:
(Some newlines added for clarity.)

$ ./bin/files cat F942
ABCDEFGHIJKLMNOPQRSTUVWXYZ
$ ./bin/files cat F942 --begin 1
BCDEFGHIJKLMNOPQRSTUVWXYZ
$ ./bin/files cat F942 --end 10
ABCDEFGHIJ
$ ./bin/files cat F942 --begin 3 --end 5
DE
$

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12071