let lemmas in fsti reported missing after val missing error 98

[briangmilnes]

I bumped into this validating Sheera's DFS, so I wrote some small test files.

• Error 98 at src/LetLemmaInFSTI.fst(1,7-1,21):
  • Some interface elements were not implemented by module LetLemmaInFSTI:
    val four_gt_three
    let three_gt_two
    If you put a val+let lemma in an fsti and don't provide the val the following let is also
    reported as not implemented which is incorrect in that the fsti provides the lemma.
    FSTI:
    /// This tests putting a let lemma in an fsti and not in the fst.
    module LetLemmaInFSTI
    val four_gt_three () : Lemma (4 > 3)
    let three_gt_two () : Lemma (3 > 2) = ()

FST:
module LetLemmaInFSTI
/// No three_gt_two.