Fix C_POLYMORPHIC_ADD resolution to C_CONCAT/C_ADD

This MR is a fix for an issue related to helping JsLIGO infer whether C_POLYMORPHIC_ADD should resolve to C_CONCAT / C_ADD:

The code

let s = String.sub(0 as nat, 1 as nat, "hello") + String.sub(1 as nat, 1 as nat, "hello")

will output

Invalid arguments.
Expected an argument of type (bls12_381_g1, bls12_381_g1) or (bls12_381_g2, bls12_381_g2) or (bls12_381_fr, bls12_381_fr) or (nat, nat) or (int, int) or (tez, tez) or (nat, int) or (int, nat) or (timestamp, int) or (int, timestamp), but got an argument of type string, string. 

because C_POLYMORPHIC_ADD gets resolved to C_ADD instead of C_CONCAT.

• has a changelog entry