It's redundant, since you can always compute them from the variable, and it
makes the code that deals with actuals rather cleaner.
On the other hand, it slightly complicates some of the tests, because any names
you use in an Actual need to be defined...
Various infrastructure too to support these.
Doing A.ForEach raised an interesting question: what does it work over? In
plain occam it'd just be arrays, but it should obviously work for lists too.
This suggests that Size and Subscript should work on lists as well, since
ForEach will be implemented in terms of them. I've therefore introduced the
idea of a "sequence" class of types.
There's obviously some overlap with the Rain typechecker here. I've tried to
cover everything in the AST that could potentially be bound into occam at some
point in the future, even if the occam parser doesn't support it yet (so
this'll do checks for Concat and mobile allocation, for example).
