Remove the errno test for now

Setting the string part of $! can't be done reliably. We'll have to figure out
another way of reporting at least the text representation of our errors.
First pass having messages be magic based
Copy message buffers when cloning new interpreters
Use the mg_free callback rather than a destructor
Stop copying data when stringifying messages

Just point the scalar's PV slot to the buffer directly, being sure to leave
SvLEN at 0, so perl won't be attempting to free our buffer.
Remove our magic when invalidating message instances
Improve the threading tests
Go back to the blessed objects containing the buffer content
Fix a bunch of warnings
A few simplifications
Merge branch 'rafl/magic'

* rafl/magic:
  A few simplifications
  Fix a bunch of warnings
  Go back to the blessed objects containing the buffer content
  Improve the threading tests
  Remove our magic when invalidating message instances
  Stop copying data when stringifying messages
  Use the mg_free callback rather than a destructor
  Copy message buffers when cloning new interpreters
  First pass having messages be magic based