Fix roundtrip check to support 5.1 <-> 5.2 migrations Printing to source code and parsing with the compiler before running the upward migration is expected to fail because on compilers older than 5.2, `fun x y -> z` and `fun x -> fun y -> z` parse to the same AST. We therefore need to first migrate it to the compiler version before printing it. Signed-off-by: Nathan Rebours <nathan.p.rebours@gmail.com>