Flip arguments of `arg` and `optionalArg`

[hendrikniemann]

Currently arg takes the type first and then the name of the argument. The inital idea was that this could be useful to quickly create alias functions like:

intArg = arg Scalar.int

In practice this is not really used and could easily be done through flip arg. Instead this is confusing because field takes the name first and in SDL also first the name and then the type is defined.