major, introduce slots for ymcw dates in union echs_instant_u

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, introduce ymcw opers FIRST/SECOND/...

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Revert "major, introduce slots for ymcw dates in union echs_instant_u"

This reverts commit f4475002e6456fb26617f4db726f110244a9ce5e.
hygiene, declare mdays[] private array globally in builders.c

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, support YMCW like dates for echs_every_year()

Syntax goes: echs_every_year(INSTANT, MAY, FIRST(MON)); for instance.

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, adapt uk holiday list to use ymcw-dates directly

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'feat/builders' into next

* feat/builders:
  minor, adapt uk holiday list to use ymcw-dates directly
  minor, support YMCW like dates for echs_every_year()
  hygiene, declare mdays[] private array globally in builders.c
  Revert "major, introduce slots for ymcw dates in union echs_instant_u"
  minor, introduce ymcw opers FIRST/SECOND/...
  major, introduce slots for ymcw dates in union echs_instant_u