Somehow protect against hugely-recursive module types

[alexcrichton]

Currently it's relatively easy to get the validator (and wasm-smith which actually generates modules like this) to go down a rabbit hole in terms of performance. To do this you can create a hugely-nested module type with expands to lots of recursive checks of subtypes:

(module
  (type $m0 (module))
  (type $m1 (module
    (import "1" (module (type $m0)))
    (import "2" (module (type $m0)))
    (import "3" (module (type $m0)))
    (import "4" (module (type $m0)))
    (import "5" (module (type $m0)))
    (import "6" (module (type $m0)))
    (import "7" (module (type $m0)))
    (import "8" (module (type $m0)))
    (import "9" (module (type $m0)))
    (import "10" (module (type $m0)))
  ))
  (type $m2 (module
    (import "1" (module (type $m1)))
    (import "2" (module (type $m1)))
    (import "3" (module (type $m1)))
    (import "4" (module (type $m1)))
    (import "5" (module (type $m1)))
    (import "6" (module (type $m1)))
    (import "7" (module (type $m1)))
    (import "8" (module (type $m1)))
    (import "9" (module (type $m1)))
    (import "10" (module (type $m1)))
  ))
  (type $m3 (module
    (import "1" (module (type $m2)))
    (import "2" (module (type $m2)))
    (import "3" (module (type $m2)))
    (import "4" (module (type $m2)))
    (import "5" (module (type $m2)))
    (import "6" (module (type $m2)))
    (import "7" (module (type $m2)))
    (import "8" (module (type $m2)))
    (import "9" (module (type $m2)))
    (import "10" (module (type $m2)))
  ))
  (type $m4 (module
    (import "1" (module (type $m3)))
    (import "2" (module (type $m3)))
    (import "3" (module (type $m3)))
    (import "4" (module (type $m3)))
    (import "5" (module (type $m3)))
    (import "6" (module (type $m3)))
    (import "7" (module (type $m3)))
    (import "8" (module (type $m3)))
    (import "9" (module (type $m3)))
    (import "10" (module (type $m3)))
  ))
  (type $m5 (module
    (import "1" (module (type $m4)))
    (import "2" (module (type $m4)))
    (import "3" (module (type $m4)))
    (import "4" (module (type $m4)))
    (import "5" (module (type $m4)))
    (import "6" (module (type $m4)))
    (import "7" (module (type $m4)))
    (import "8" (module (type $m4)))
    (import "9" (module (type $m4)))
    (import "10" (module (type $m4)))
  ))
  (type $m6 (module
    (import "1" (module (type $m5)))
    (import "2" (module (type $m5)))
    (import "3" (module (type $m5)))
    (import "4" (module (type $m5)))
    (import "5" (module (type $m5)))
    (import "6" (module (type $m5)))
    (import "7" (module (type $m5)))
    (import "8" (module (type $m5)))
    (import "9" (module (type $m5)))
    (import "10" (module (type $m5)))
  ))
  (type $m7 (module
    (import "1" (module (type $m6)))
    (import "2" (module (type $m6)))
    (import "3" (module (type $m6)))
    (import "4" (module (type $m6)))
    (import "5" (module (type $m6)))
    (import "6" (module (type $m6)))
    (import "7" (module (type $m6)))
    (import "8" (module (type $m6)))
    (import "9" (module (type $m6)))
    (import "10" (module (type $m6)))
  ))
  (type $m8 (module
    (import "1" (module (type $m7)))
    (import "2" (module (type $m7)))
    (import "3" (module (type $m7)))
    (import "4" (module (type $m7)))
    (import "5" (module (type $m7)))
    (import "6" (module (type $m7)))
    (import "7" (module (type $m7)))
    (import "8" (module (type $m7)))
    (import "9" (module (type $m7)))
    (import "10" (module (type $m7)))
  ))
  (type $m9 (module
    (import "1" (module (type $m8)))
    (import "2" (module (type $m8)))
    (import "3" (module (type $m8)))
    (import "4" (module (type $m8)))
    (import "5" (module (type $m8)))
    (import "6" (module (type $m8)))
    (import "7" (module (type $m8)))
    (import "8" (module (type $m8)))
    (import "9" (module (type $m8)))
    (import "10" (module (type $m8)))
  ))

  (type $m (module
    (import "" (module (type $m9)))
  ))
  (import "a" (module $a (type $m9)))
  (import "b" (module $b (type $m)))
  (instance (instantiate $b "" (module $a)))
)

Today that module takes quite a long time in validation (30s!).

I'm not entirely sure how to prevent this just yet, but we shouldn't in any case have modules take 30s in validation when they're so small. Whatever fix is applied here needs to be extended to wasm-smith since sometimes generating modules takes 30+ seconds since it's doing subtyping checks internally.