Change pure Int constants into @enums (where possible)

There are a few places where numeric constants should be represented as @enums instead of Ints.

Important note: This is not possible now in all cases because of a restriction with Haxe 3.1.3 which essentially prevents the handling of enum values that are unknown at compile time. Thus, enum values that may be subject to further expansion (such as exception codes) should not be converted unless Haxe 3.1.3 support is dropped.