Template: Don't assume mapping is enumerable

[Hbutlercapone]

this allows template expansions with lambdas or anything else that responds to []

[therabidbanana]

https://github.com/sporkmonger/addressable/pull/133#issuecomment-25284634

This point was made with an earlier request for this type of feature - I think we probably want to add some additional checks to make sure the API is solid and add a bit more to the docs?

[therabidbanana]

An additional commit that covers the requested doc updates, argument errors, and adds more tests for edge cases from this particular update (partial_expand doesn't make sense with a lambda).

I can set up a new PR with the changes or you can cherry pick my commit into your PR @Hbutlercapone and clean them up some more if you'd like - I don't know how we'd want partial_expand to work here but there's a few ways we could approach it aside from "throw error"

https://github.com/sporkmonger/addressable/compare/main...therabidbanana:addressable:pr/133

[Hbutlercapone]

I will hop on after work tonight and take a look!