Implement required attribute (swift)

[jnordberg]

This implements the (required) schema attribute in swift by force-unwrapping the properties. Not sure if this is the best way to go about it but it works for my use-case.

I couldn't get the test suite to run (got a bunch of weird syntax errors and missing modules for maxim.zaks.flatBuffers.tests, is there some additional code generation that has to be run there?) so there is probably some cases I haven't covered yet.