Emit metadata as comments

[MattWindsor91]

Something I've been wanting to do for a long, long time is make it so that the reifier can output comments representing metadata. This would be purely for debuggability, as the parser/abstractor wouldn't be able to read them back in.

I think the way this would work would be:

• reifiers that touch metadata would acquire an optional argument ?reify_meta:('m -> string option) with a default being fun _ -> None;
• the AST would acquire new statement and expression types that prefix other statements/expressions with comments (which would get deleted by abstraction);
• the pretty-printer would output comments wherever these appear.

[MattWindsor91]

One issue may be that I think the filtering boilerplate for the fuzzer assumes that it's converting to and from the same metadata type (unit), but this might not be true.

[MattWindsor91]

This ended up eating an entire day of work without any actual progress, so I'm a little averse to working on it again in the near future...