as_xyz accessors should return Result

[rcoh]

Currently, as_xyz accessors return Option<T>. But this can be confusing for users when the type is actually a different type. Instead, as_xyz should return Result<&T, &Self>:

Proposal:

    pub fn as_ss(&self) -> Result<&std::vec::Vec<std::string::String>, &Self> {
        if let AttributeValue::Ss(val) = &self {
            Ok(&val)
        } else {
            Err(&self)
        }
    }

This should make the problem immediately clear and easy to debug.

[robisonsantos]

On the same methods, it would be nice if the method "respected" the type from dynamo and returned the proper representation. For example, as_n() is always handling a number, as_b() is always handling a boolean, etc.

If we take the "principle of least surprise", I'd expect as_n to return a Result<float, &Self>, as_b to return a Result<bool, &Self> etc.

Consumers of the API should not need to parse and transform data types read from dynamo if those types are well known.

[rcoh]

This is a little trickier but may still be feasible—what should the rule be? I guess we could auto-unwrap through structures with one field?

Normally backwards compatibility problems would prevent this but unions are actually much more stringent

[rcoh]

This is fixed in #527