Sexp converters for Cstruct and improved bounds checking

• Add bounds checks for Cstruct.BE/LE functions that violate a view.
  Previously, only bounds errors on the underlying buffers would raise.
  Bug #25, reported by Mindy Preston in mirage/mirage-tcpip#56.
• Add 'Lwt_cstruct.complete' to ensure that read/write operatiosn
  run to completion.
• Add Sexplib conversion functions to Cstruct.t values (#27 #22).