v0.2.0

A minor rather than a patch: new behaviour toward the consumer's own code
(an ignored arity report, a newly typed matcher) and a raised dependency
floor are both boundaries Composer's caret already treats as breaking on 0.x.

• The rasuvaeff/understudy floor rises to ^0.4: the fixtures that prove
  the new behaviour are written in the 0.4 idioms, and a lowest-versions run
  against 0.1 would be proving nothing. Consumers on an older understudy stay
  on the 0.1.x line of this package.
• understudy 0.4 idioms (#6). Arg::rest():
  the arguments.count report is ignored (via an IgnoreErrorExtension)
  wherever a call's last written argument is Arg::rest(); in a real call
  the line is reported once, as understudy.matcherLeak — the leak is the
  actual mistake, and the engine answers the call with ArgumentCountError.
  Arg::captor(): $captor->capture() is typed never like the Arg::
  factories when the receiver is a Captor, no longer counts against
  "exactly one call per closure", and is reported by understudy.matcherLeak
  when it reaches a real call; Arg::captor() itself stopped being collected
  as a matcher — it is the factory, legitimately outside any specification,
  and was falsely reported as a leak.