Error immediately on cycles in attribute evaluation

[krame505]

Changes

This tracks whether an attribute has been demanded on a DecoratedNode, to immediately report an error when encountering a cycle, rather than crashing with a StackOverflowError.

Documentation

Added doc comments.

[krame505]

Turns out that checking for a cycle on every thunk is actually more efficient than doing it for syn/inh accesses on DecoratedNode. This should also catch cycles that don't involve attribute accesses (e.g. local x::Expr = neg(x);.)