Allow Enumerable for query args instead of Array

[lwakefield]

This is a little bit of a "see if it sticks", in the sense that I don't have a full grasp of what might break as a result of this proposed change (even though specs pass!)

The impetus for this change, is getting named bindings to work. On top of these changes, I can now do the following:

class SQLite3::Statement
    private def bind_arg(index, tuple : Tuple)
        k, v = tuple
        idx = LibSQLite3.bind_parameter_index(self, ":"+k.to_s)
        bind_arg(idx, v)
    end
end

db.query_all("select 'hello '||:msg", args: {:msg => "world!"}, as: {String})

So - WDYT?

[bcardiff]

Regarding lifting Array to Enumerable it seems time has come #110 (comment) :-)

On the named argument api though I am not so sure if treating Tuples like named argument is the accurate thing. Sure is convenient, but what if you want to send an actual tuple as value? An alternative would be to have a separate record NamedArgument which can be implemented by each driver. The api become less attractive though

db.query_all("select 'hello '||:msg", args: [NamedArgument(:msg, "world!")], as: {String})

Maybe for convenience we can have a named arg named_args that is implemented on top of the above construct (or not).

db.query_all("select 'hello '||:msg", named_args: {msg: "world!"}, as: {String})