Fix quoting and unquoting of primitive strings

[MathisBD]

This PR fixes the following bug related to quoting primitive strings :

From MetaCoq.Template Require Import All.
From Coq Require Import PrimString.
MetaCoq Test Quote "hello"%pstring.
(* produces (String.String "hello"%pstring) which is ill-typed. *)

The bug comes from the fact that in template-coq's quoting/unquoting code, the ocaml variable named tString is used to refer to two distinct Coq terms :

• the second constructor of bytestring.string.
• the primitive string constructor of term.

The fix involves changing the names of a couple ocaml variables : see the diff for more info.

[yforster]

Could you add a file in the test-suite which checks this behavior? I.e. add the code snippet from the PR as file there?

[yforster]

Thanks! Once CI goes green I'll squash and merge and cherry-pick the commit over to coq-8.20.