MVar: initial implementation.
Dataflow: add missing example in derivation.
Merge pull request #18 from chrisseaton/dataflow

Dataflow: add missing example in derivation.
MVar: full?
MVar: #modify.
MVar: non-blocking variants.
MVar: documentation.
MVar: include Dereferenceable.
Merge pull request #17 from chrisseaton/mvars

MVar: initial implementation.