Normalise RakuAST class specifications (4/N)

In the sense that all roles are mentioned *under* the class specification,
each on their own line.  This at least makes reading these a lot easier
for *me*.

No functional changes.  Please revert if you think it's a bad idea.

src/Raku/ast/compunit.rakumod

@@ -1,7 +1,11 @@
 # A compilation unit is the main lexical scope of a program.
-class RakuAST::CompUnit is RakuAST::LexicalScope is RakuAST::SinkBoundary
-                        is RakuAST::ImplicitLookups
-                        is RakuAST::ImplicitDeclarations is RakuAST::AttachTarget {
+class RakuAST::CompUnit
+  is RakuAST::LexicalScope
+  is RakuAST::SinkBoundary
+  is RakuAST::ImplicitLookups
+  is RakuAST::ImplicitDeclarations
+  is RakuAST::AttachTarget
+{
     has RakuAST::StatementList $.statement-list;
     has RakuAST::Block $.mainline;
     has Str $.comp-unit-name;