Add SequentialType (RedHat AST’s SequenceType)

I changed the name for two reasons:

1. It stands for Sequential, not Sequence; the name should reflect that
2. This disambiguates it from SequencedType (which is completely different)
RedHatTransformer: add missing tokens
source-gen: Fix equals indentation

This is exactly why I want to have ceylon.ast + ceylon.formatter based
source-gen. These kinds of whitespace issues. They’re annoying.