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++.
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