Duplication of type quantifiers when extracting models

[br4sco]

Compiling the following model

include "seq.mc"
let model: () -> Float = lam.
  foldl2 (lam acc. lam x1. lam x2.
            addf acc (assume (Beta x1 x2)))
         0.0 [1.0, 2.0, 3.0] [1.0, 2.0, 3.0]
mexpr
model ()

with cppl will duplicate the type quantifiers of foldl2 defined in seq.mc to something similar to

let foldl21: all c. all b7. all a28. all a28. all b7. all c. Unknown

in the resulting mexpr program. The problem seems to arise in the extraction phase of model compilation. Note that the output program will also fail to type check due to miking-lang/miking#830 when using partial CPS. However, the quantifier duplication also occurs for full CPS and no CPS.