Fix clone of embedded literal sql

[prust]

This fixes #118, with a couple of tests to protect against regressions.

The docs state that sql() should be used:

somewhere that a value is expected (the right-hand side of WHERE criteria, or insert()/update() values)

and in these situations, clone() was working correctly because it doesn't clone values (which are typically immutable, though I suppose it's technically possible for someone to mutate a sql() literal).

But right after that, the docs give an example where sql() is used in a higher-level context -- not just as a value, but as the entire WHERE expression. It is in these cases that it was throwing the error, and this is what was fixed in this pull request.

I should probably update the docs to explicitly allow sql() in more contexts and should think a little harder about whether we really need/want that functionality in the big 3.x refactor (which may never happen, at this rate).