Converted ToSql usage to impl IntoIterator

[kaibyao]

Addresses #265 and #445. This is about as far as I could get with me being new to Rust.

There's still issues:

• I'm not happy that empty params (&[]) has to be explicity cast to &[some type that implements ToSql (like i32)]. Example: &[] as &[i32]. I'd prefer being able to use the existing method of just sending an empty &[], as that would cause no breakage with existing usage.
• Also, test test_bpchar_params fails because I don't know how to work with None::<&'static str>.
• I haven't figured out the syntax for sending parameters that contain different types.

@sfackler, could you take a look at this and see how we might be able to fix these things?

[sfackler]

The type inference regressions in things like &[] is pretty prohibitive on making this change IMO. One option would be to keep the current methods and add new ones that take iterators.

[kaibyao]

I agree; closing this for now as it's not ideal.

[norcalli]

The solution for the &[] empty case is typically to make a utility function which has a concrete type such as fn empty() -> &'static [u64; 0] which users can call as .execute(query, empty()). Considering 0.16 is still in RC mode, would it be something that could go in that?