[New module system] Type/constraint synonyms always warn as unused

[m-yac]

In #1363 type and constraint synonyms seem to always generate unused warnings. For example, in the REPL:

Loading module Cryptol
Cryptol> type X = [8]
Cryptol> 5 : X
[warning] at <interactive>:1:6--1:7
    Unused name: X
0x05

This can also be seen in tests/examples/allexamples.icry.stdout, for example.

Like #1371, this appears to be a renamer bug but I wasn't able to find anything after a quick first pass.

[m-yac]

The issue in allexamples.icry ended up being specific to functor instances. A more minimal example is:

interface module I where
  type n : #

module A where
  import interface I
  type T = 5

module M where
  submodule N where
    type n = 7
  submodule B = A { submodule N }
  import submodule B as B
  type MyT = B::T

Compare this to the following example:

module M where
  submodule B where
    type T = 5
  import submodule B as B
  type MyT = B::T

Recall that when warnUnused is called, it generates a warning for each name which has exactly one usage counted in the current module. In both examples, the declaration type MyT = T adds one to the usage count of B::T in M. In the second example the declaration of T in the submodule B also adds one to B::T's usage count in M, but in the first example there is no such declaration. Thus, the first example generates an unused warning for B::T (rendered confusingly as an unused warning for T in A.cry, since the location of B::T remains in A.cry) but the second example does not.

This is fixed in 2c179b1 by keeping track of which names came from functor instances, and ignoring them in warnUnused. I did this by simply adding a new constructor to the new OrigSource datatype, which already keeps track of whether a name came from a definition or a module parameter.

One idea I did not implement was simply adding one to the usage count of every definition when instantiating a module. This change would mean requiring the user to use every definition in an instantiated module if they want no warnings, which doesn't seem like what we want.