Java2Swift: Factor Java class translation into its own type & file

The main function for translating a Java class into a sequence of Swift
declarations was very large and very tangled. Refactor it into a separate
class that more deliberately separates the analysis of the Java class
(picking out what needs to be translated) from the actual rendering of
the Swift declarations, then breaks that rendering down into more
digestible pieces.

No (intended) functionality change here: this is cleanup to make it
easier to implement some upcoming improvements.

Author: DougGregor