This changes the indexing syntax from .() to [], the vector syntax from ~[] to [] and the extension syntax from #fmt() to #fmt[]
While it is still technically possible to test stage 0, it is not part of any of the main testing rules and maintaining xfail-stage0 is a chore. Nobody should worry about how tests fare in stage0.
In 'rec(a = b with c)', if a exists in c then the expected type for b is the type of c.
