Add summary statistics about different proofs

[solna86]

I found this repository incredibly interesting.

I think it would be very informative to extract some kind of naive summary statistics (e.g. LOC) to understand the compromises between different proof systems.

As an old user of Spec#, Dafny's precursor, I was pleasantly surprised to see how well Dafny and other systems (e.g. Lean, Agda, F*, Coq/SSReflect or Idris) compare to Isabelle or Liquid Haskell.