Give a proper type to literal Strings

[paulcadman]

Previously literal strings were assigned the magic type {A : Type} -> A. Now we have the BuiltinString type we can use that to type literal strings instead.

It also means we can now evaluate string literals in the REPL:

Juvix REPL version 0.2.8-ff748b9: https://juvix.org. Run :help for help
OK loaded: /Users/paul/heliax/juvix-2023/.juvix-build/stdlib/Stdlib/Prelude.juvix
Stdlib.Prelude> :t "a"
String
Stdlib.Prelude> "a"
"a"

Fixes #1720

[janmasrovira]

Awesome, thank you