PSA spec replace "instantiation tree" with "instantiation graph"

[jafingerhut]

Section 7.1 "Restrictions on where externs may be used" describes an "instantiation tree". The problem is that while many P4 programs are written such that this graph is a tree, it is possible to write P4 programs where it is not restricted to be a tree, e.g. the one in the P4_16 language spec section 17.2 "Compile-time Evaluation".

I believe that with the P4_16 language specification versions 1.0 through 1.2, it is possible to prove that the instantiation graph must be a directed acyclic graph, which might be useful to describe somewhere. I suspect that detail is more of interest to P4 compiler writers than anyone else.