v0.9.0

• Breaking: Gen::oneOf() and Gen::elements() reject an
  ArbitraryInterface among their values. oneOf(generator, generator) is how
  fast-check, jqwik and Hypothesis spell "pick one of these generators", and
  here it made the generator objects themselves the data: the body received
  IntArbitrary and StringArbitrary instances, never a generated value, and
  the property reported green having checked nothing. Pick between generators
  with Gen::frequency(); oneOf/elements take values.
• Breaking: Gen::regex() / Gen::stringMatching() reject a delimited
  pattern. The compiler always wanted [a-z]{3,6}, not /[a-z]{3,6}/, and the
  word "delimiter" appeared nowhere in the code or the documentation. A PHP
  developer arrives with a pattern from preg_match(), which is always
  delimited, and got one of two things: an anchored pattern was blamed on its
  anchor (the ^ is not leading only because a / precedes it), and an
  unanchored one compiled with the delimiters as literals — values that still
  passed an unanchored preg_match(), so the property stayed green while every
  generated string carried two junk characters. The rejection names the
  undelimited pattern to paste back, and escaping the character still matches
  it literally.
• Environmental skips no longer spend the discard budget, and
  RunStatistics::$skips counts them apart from $discards. A skip says
  nothing about the input, so charging it to the discard budget made a machine
  missing a dependency give up — and be advised to narrow generators that were
  never at fault. GaveUpException gained $skippedRuns and
  $exhaustedBySkips, and reports the environment rather than the generators
  when it is the skip budget that ran out. Both budgets share the
  maxDiscards cap.