Hello all,

I've attached an improved (but still preliminary) implementation of
MVars for Guile.  Apart from fixing some bugs, this version follows the
Haskell API and semantics more closely.  In particular, I now refrain
from adding atomicity guarantees beyond those promised by the Haskell
API.  I also added locking to 'mvar-empty?', to ensure that we meet the
"ordering" requirement of the Haskell API.

Comments and suggestions welcome.

      Mark