Point two of #998

• Extract CatchParameter from FormalParameter
• Introduce UnionType to represent multicatch types, eg IOException | AssertionError. This is just a trick (the Java type system doesn't feature union types), but it makes it so that each VariableDeclaratorId has either exactly one type node, or zero if the type is inferred.
• Cleanup the API of ASTCatchClause:
  • Remove getCaughtExceptionTypes, getExceptionName, getCaughtExceptionTypeNodes, isMulticatchStatement
  • All those are now accessible on the CatchParameter and we don't need to duplicate them

TODO on master

• Deprecate ASTCatchStatement#getBlock in favour of getBody