Make `with` clauses take a bind and an argument

[developedby]

Currently we write

fold x with a:
  ...

But since with only takes variable names, we are forced to declare the state variable a earlier, which is annoying

Instead we want to be able to do

fold x with a=0:
  ...

This change would also simplify the compiler a bit, since with is different from everything else, since it's the only of these clauses that takes only a name and not a value.