Log Message:
unison-snapshot: Update to 2.51.70

This is an alpha for 2.52.0 and contains significant changes.

While just upgrading and using it should work, it has not received
enough testing to be called beta.  In particular the new formats have
not been tested with big endian processors.

## Changes in 2.52.0

   * Feature negotiation, compatible with 2.51.
   * New archive format (independent of ocaml version, based on umarshal)
     Upgrade is automatic.
   * New wire protocol (independent of ocaml version, based on umarshal)
     New protocol is used if both sides are >= 2.52.0.
   * Support for unix-domain sockets
   * Many bugfixes and minor improvements
   * ocaml compatibility is now >= 4.01
   * NEWS is now in NEWS.md and not in the manual