rfind bugfix: handle empty pattern like string.find

Dependencies

• [2] 7PZ4CQFV search: transparently handle drawings everywhere
• [3] BBLXANWI add some tests for rfind
• [*] LXTTOB33 extract a couple of files

Change contents

• edit in search.lua at line 134
  [2.617]

  [5.51793]
  + if #pat == 0 then return #s end
• edit in search.lua at line 147
  [3.38]

  [3.38]
  + check_eq(rfind('abc', ''), 3, 'empty pattern')