Pretend we use bigints for Sexp integers

* btl/builtins.typer: Slight reorganisation.
(Float->string): Rename from Float_to_string.
(Int->Integer): Give it a type.
(Sexp_integer): Change its type to be `Integer`.
(Sexp_dispatch): Make matching change.

* src/eval.ml: Add new "Int->Integer" primitive.
(make_integer): Make it take a bigint.
(sexp_dispatch): Make it use a bigint for the integer case.