Handle int64 and IntLit

[NathanReb]

let json = [%yojson 1]

is rewritten as

let json = `Int 1

which is fine but in some case we would like to be able to have it rewritten to:

let json = `Intlit "1"

because when using ppx_deriving_yojson for instance, some small int values will still be converted to Intlit, as would [%to_yojson: int64] 1L.

A good first step is probably to accept such litterals and the payload and rewrite them as Intlit, ie:

let json = [%yojson 1L]

gets rewritten as the above example.