By popular demand, read effects are back. They serve to mark dependence
of a program function on external variables which are otherwise not
mentioned in the function's specification. Such annotation is necessary,
for example, to add the needed type invariants.

The "reads" clauses are comma-separated variables (contrary to write
effects, where one must point out the modified field).

If a user specifies a "reads" clause for a defined function, we check
that every listed variable occurs in the code and that every free
variable in the code occurs in the specification (which includes the
"reads" clause). Notice that this concers function arguments, too:

  val r : ref int
  let f (x : ref int) reads {r} = x := !r

would require x to be added to reads.

One exception to this rule is the the result type in a non-recursive
function may contain regions coming the function's arguments (though
not from the context). It is sometimes useful to write a function
around a constructor or a projection: see function [create] in
verifythis_fm2012_LRS.mlw. For recursive functions we impose
the full non-alias discipline.

Now that ghost writes are checked via e_syms.syms_pv,
there is no use for read effects anymore.

This fixes one bug and several deficiencies in the previous
implementation, which was based on read effects. One caveat
about the current version is that we treat all type variables
in a polymorphic type as visible (= non-ghost) even if they
only occur in the ghost fields. For example, the following
code is rejected:

  type strange 'a = { ghost contents : 'a }

  let test () =
    let x = { contents = ref 0 } in
    x.contents := 42

Even though the field [contents] is ghost and cannot be reached
from the real-world code, the type of x is [strange (ref int)]
and the region of the reference is considered to be visible.
Thus the last assignment is seen as an illicit ghost write.

On the other hand,

  type strange 'a = { ghost contents : ref 'a }

and

  type strange 'a = { ghost mutable contents : 'a }

permit ghost writes into [contents] without problem.
Ghost writes into ghost variables are also okay.