Linearity check for exist bindings

[rachitnigam]

When a component defines an existential variable, it must provide exactly one exist binding in every scope. For example, the following program is malformed:

comp Foo[A](...) with {
  exists L;
} {
 if A > 10 {
   exists L = 10
 }
}

If A <= 10, then when monomorphizing, the module will not have a binding for L which will cause compilation failure.

It should be relatively easy to write a well-formedness pass that ensures that all scopes of a program assigns a value to each existential parameter exactly once.