The existing methods which take slices of parameters work well when
directly passing a temporary slice (e.g. `c.query(s, &[&15, &"hi"])`,
but becomes limiting in other contexts like when programmatically
building up a query. We now offer methods which take iterators, which
are significantly more flexible for these kinds of use cases.
Because of the weird object safety of `ToSql`, we can't be generic over
`Iterator<Item = impl ToSql>`, but instead have to specifically work
with `Iterator<Item = &dyn ToSql>`. This may require a `.map()` or two
but should still work fine.
Closes#265
8192c771c0