Add Ast node(s) representing comments

[hamnis]

Sometimes it is very useful to be able to add comments to generated code when using scalameta.
Currently this is not possible.

val instances: List[Stat] = cases.map(c => q"""
           // ${c.comment}
           case object ${c.name.termName} extends $init""")

/*
can't unquote into single-line comments
          ${c.comment}
*/

This is the same with multi-line comments.

[tgodzik]

Thanks for raising this! As mentioned on discord there was a previous discussion at #1137 (comment)

It seems that having comments in quasiquotes is most problematic, we could do attachements or special AST nodes that woul pretty print correctly.