git pkt-line format

Git uses pkt-line in the pack and http protocols, and for the long-running
filter processes protocol as well.

This should be a quite efficient parser and builder since it uses
attoparsec and bytestring-builder.

This adds a dependency on attoparsec, but it's a free dependency because
eg aeson depends on attoparsec and git-annex depends on aeson.

This commit was supported by the NSF-funded DataLad project.