source-gen: Split up Generator.ceylon

Mostly done using the IDE’s awesome “move to new source file” proposal.
(This is how I found ceylon/ceylon-ide-eclipse#1022.)
source-gen: Specify alias cases in multiple arguments

Changes the syntax of alias generation from

    ./alias Name 'Case1|Case2|Case3'

(quoting necessary to avoid shell piping; just generally annoying) to

    ./alias Name Case1 Case2 Case3

(We can’t do this for abstract classes because they can have members.)
source-gen: improve ConcrClassGen.genBackendTest

* now adds a construct() method that builds up test cases from other
  test cases
* now adds a default test case
NarrowingTransformer: fix order of methods

source-gen inserts the PrecedenceXExpression methods in the wrong place,
since it doesn’t compare numerically. Seems I forgot to fix it manually
for these two methods.
RedHatTransformer: fix method order

transformLogicalOperation was in the wrong place.
fixup! source-gen: improve ConcrClassGen.genBackendTest
Add ThenOperation, ElseOperation (RedHat’s ThenOp, DefaultOp)
Add tests for ThenOperation, ElseOperation