Scala support

[Synesso]

This continues on from the good work started by @nbauernfeind and supercedes #32. At the time of opening the PR, the compilation errors against test fixtures have been resolved. Remaining work:

• Resolve compilation warnings about case-insensitive classnames
• Test validity of sources generated against https://github.com/stellar/stellar-core/tree/master/src/xdr

[Synesso]

I've added union test case scenarios that arise when generating Stellar objects and were not already covered:

• Union nested unions
• Unions with void default arms
• Enum discriminated unions with incomplete arms and no default
• Type aliased primitive discriminated unions

[Synesso]

This is ready for review.