Make `indexmap` dependency optional

[zeenix]

Sometimes one just needs parsing/deserialization and it would be great to be able to do that with this crate w/o having to drag indexmap and its deps along only for that (See bkchr/proc-macro-crate#37). I believe the order is only important when writing back data after reading it but not, when only parsing so this should be possible?

[epage]

I experimented with this for a bit but ran into some problems but can't remember what they were.

#808 might allow us to drop it regardless.

[epage]

Hmm, we made ourselves further entrenched into indexmap by using the Raw API.