…m now works

  * This PR was motivated by Issue scala-native#1673 " regex RE2 wrongly ignores
    newline in OR groups".

    The defect was a botched rewrite of the Parser.scala method
    matchRune() in an attempt to remove multiple `return` statements.

    The issue is now fixed for the two patterns which would evoke
    the presenting problem.  The patterns "\\n|." and ".|\\n"
    are both used to indicate a match to ANY_CHARACTER, including
    newline. Because regex is defined to have a preference for the
    left term, they execute slightly different code paths.

  * Two unit-test cases were added to ParserSuite.scala. Each tests
    one of the two patterns which evoked Issue scala-native#1673.

  * Details for future maintainers.

    + The FLAGS argument to testParseDump() in NOMATCHNL_TESTS as 0.
    That is, all flags are clear. This mean MATCH_NL is clear, which
    matches Java 8 DOTALL being clear: dot does not match newline.

    Most of rest of Suite uses FLAGS which match PERL:
   `TEST_FLAGS = MATCH_NL | PERL_X | UNICODE_GROUPS`
    This sets dot to match newline and does not evoke Issue scala-native#1673.

    + Before this PR, Parser.scala with MATCH_NL clear compiled uses of
      the patterns to "dnl{}"; that is, ROP.ANY_CHAR_NOT_NL.
      As of this PR, the patterns get compiled to the correct and
      expected "dnl{}", ROP.ANY_CHAR. See NONMATCHNL_TESTS .

Documentation:

    * The standard changelog entry is requested.

Testing:

  * Built and tested ("test-all") in debug mode using sbt 1.2.8 on
    X86_64 only . All tests pass.