Update to parrot version 4.2.0.

Upstream changes:

- API Changes
    + The signature of getprop was changed from (PMC,String,PMC) to
      (PMC, PMC,String) for consistency
- Core
    + Parrot Calling Conventions (pcc) now reuses Continuation PMCs
      internally, which reduces GC work by 25% and improves
      the fib.pir benchmark by 6%
    + Winxed snapshot updated to 1.6.devel 44a04cfa7b
    + Improved the detection of Clang-ish compilers during configuration
    + Fixed a possible segfault bug when reading packfiles with no
    constants or main_sub
    + By default, Parrot has now elevated these GCC warnings to errors
      during compile time:
        implicit-function-declaration, undef, missing-braces,
        nested externs, old-style-definition, strict-prototypes,
    + The OS Dynamic PMC now has separate functions to unlink a file
      and remove an empty directory (rmdir)
    + Fix building on Cygwin due to an improperly named DLL file
    + Various small bug fixes pointed out by static and dynamic analysis
      tools
- Branches
    + Work on M0 continues now in the m0 branch, which contains both
      implementations (currently C and Perl) and specification.
    + Good progress has been made on the threads branch which builds
      on the green_threads branch. This gets Parrot much closer to
      being able to utilize multiple CPU cores seemlessly. More details
      at http://niner.name/Hybrid_Threads_for_the_Parrot_VM.pdf
- Documentation
    + New release manager documentation for parrot.github.com :
        http://git.io/parrot-github-guide
- Community
    + Parrot was accepted to Google Summer of Code 2012!
      Ideas Page: http://git.io/parrot-gsoc-2012

(he)