Merge branch 'en/xdiff-cleanup-2' into seen

Code clean-up.

Comments?

* en/xdiff-cleanup-2:
  xdiff: rename rindex -> reference_index
  xdiff: change rindex from long to size_t in xdfile_t
  xdiff: make xdfile_t.nreff a size_t instead of long
  xdiff: make xdfile_t.nrec a size_t instead of long
  xdiff: split xrecord_t.ha into line_hash and minimal_perfect_hash
  xdiff: use unambiguous types in xdl_hash_record()
  xdiff: use size_t for xrecord_t.size
  xdiff: make xrecord_t.ptr a uint8_t instead of char
  xdiff: use ssize_t for dstart/dend, make them last in xdfile_t
  doc: define unambiguous type mappings across C and Rust