A symbol is now considered overloadable if it satisfies
the following conditions:
  - it has at least one parameter
  - it is non-ghost and has fully visible result
  - all of its parameters are non-ghost and have the same type
  - its result is either of the same type as its parameters
    or it is a monomorphic immutable type.

An overloadable symbol can be combined with other symbols of the
same arity and overloading kind. Otherwise, the new symbol shadows
the previously defined ones.

This generalisation allows us to overload symbols such as "size"
or "length", and also symbols of arbitraty non-zero arity.

I am reluctant to generalize this any further, because then we
won't have reasonable destructible signatures for type inference.

This reverts the enforcing part of a44cccbb.

This reverts commit da67461f.

Let us keep the {..} notation for snapshots and shadowed
logical symbols, but go back to transparent access to logical
symbols, at least for now.

- requires a lot more testing
- support in extraction missing (exception raised)
- interaction with "syntax literal" remains to investigate

- test-ocaml-extraction is back (and fixed to resolve overloaded +)
- exec/extract tests that have been moved to to_port are commented out

x.{f} is allowed and can be used for unary applications
M.{f} is not allowed, use {M.f} instead

It is confusing to use the keyword "label" to define an exception.
Also, the "label L in ..." binds too far and the break point is
not clearly defined.

We do need some syntactic sugar for
  exception X t in try <expr> with X r -> r
(at the very least "break" and "continue"), but labels are not it.