This is mostly straightforward: modify the parser to allow direction
decorators in the right places, and extend the type checker to match.
There's some slight awkwardness in that some of the Types functions
have to perform the same checks as the type checker (e.g. directing a
non-channel), so I've tidied up their error messages a bit.
At the backend, I've just added a little pass to strip out all the
DirectedVariables, since the other backend passes don't handle them
gracefully. From the occam/C point of view this is fine, but I'm not
sure if it's going to cause problems for C++.
This fixes the AST, parser and typechecker, and adds a pass to
transform Result back into Abbrev, but doesn't transform Initial yet.
(It actually works for trivial stuff anyway, but it won't do the right
thing for complex types or PROC parameters.)
It appears (to me) to make sense to support INITIAL/RESULT reshaping
and retyping too, so this does.
Refs #42.
