Fixes #1691 (and removes partial match warning)

[yav]

No description provided.

[RyanGlScott]

LGTM, aside from some minor spelling and style suggestions.

[RyanGlScott]

I generally prefer enumerating the data constructors instead of using a wildcard pattern, as this makes it harder to forget this code later if we ever change the definition of MaybeAnon. I'll defer to your judgment on which is better, however.

[RyanGlScott]

A similar comment about wildcard patterns here.

[RyanGlScott]

Also, there is a test suite failure on Windows:

      issue1455_3.icry: [Failed]
21,28d20
< Submodules
< ==========
<     
<   From Inst3
<   ----------
<        
<     import of F at issue1455\G.cry:5:1--5:9
<      
278,283d269
<      
<   From Inst3::import of F at issue1455\G.cry:5:1--5:9
<   ---------------------------------------------------
<        
<     a : [4]
<     b : [8]

# If output is OK:
cp D:\a\cryptol\cryptol\output\.\tests\issues\issue1455_3.icry.stdout \
     D:\a\cryptol\cryptol\tests\issues\issue1455_3.icry.stdout.mingw32