using references in the public API

[OmarTawfik]

Incoming

Can we accept a reference to the source string in Language::parse() instead of owning it? using shared memory?
Is it worth it to avoid the copies in case of large strings?

Outgoing

Many API calls currently involve cloning and returning an Rc<T> needlessly, rather than returning &T. We should review and convert them where needed.

• More context: Add Tree Query Language and Engine #753 (comment)

[AntonyBlakey]

Many API calls currently involve cloning and returning an Rc<T> needlessly, rather than returning &T. We should review and convert them where needed.

Not sure about that. We should maybe return &Rc<T>, to enable sharing, which you cannot do with a raw reference.