Check for Static Field Assignment

[DavePearce]

This should not compile:

int GLOBAL = 0

function f():
    GLOBAL = 1

[DavePearce]

Have implemented this, but it remains to handle these cases:

int x = 0

method main():
   assert x == 0

Currently above does not compile, though it seems reasonable to assume that it would.

[DavePearce]

Presumably a loop invariant would also be able to access it.

[DavePearce]

Also this should be valid I suppose as well:

int var = 1

method m()
requires var >= 0:
   ...