Merge branch 'next'

* next:
  test, adapt uk holiday list to include William+Kate wedding
  minor, provide echs_stream() builder to inject single explicit events
  test, implement blockers for xmas mover in uk holiday DSO through echs_every_week()
  minor, provide echs_every_week() builder
  minor, provide more effective movers using a blocking-interval
  hygiene, use *x = *y cloning in __mov_ctl() and calloc() (instead of malloc() closure) in _move()
  minor, make day part and intraday part of echs_instant_t objects available separately (through .dpart and .intra)
  minor, provide echs_instant_diff() and echs_instant_add() as well as a struct for instant diffs (echs_idiff_t)
  minor, provide some useful event fiddling routines in event.h
  minor, adapt uk holiday list, using echs_move() to move christmas and boxing day
  minor, provide echs_move_{after,before}() to implement in-lieu-of feature
  minor, provide muxer macro ECHS_MUX() for convenience