CeylonExpressionTransformer: Improve Identifiers

We will now print the ‘usePrefix’ flag only if it’s true.

For #39.
CeylonExpressionTransformer: Improve sequences

Only put the sequence in a single line if the content is a single line.
This avoids stuff like this:

    elements = [ImportTypeElement {
            name = ...;
	    nestedImports = ImportElements { ... };
	}];

For #39.
completeCompilationUnit: Add imports

They even make sense! That won’t hold for the rest of the CU, though…