Improve support code (as_value/as_list_value)

[pjenvey]

Looking forward a bit into an improved high level Spanner client, our as_value and as_list_value methods could be improved.

They provided an easy way to support both client sdks simultaneously, but we could improve how types are converted to protobuf Values via a trait, similar to diesel's ToSql trait.

Say a ToSpannerValue trait with impls for basic types like String, i32 (and in our specific case, SyncTimestamp). And an impl for either Vec<V> or possibly a fancy one like T: where T: Iterator<Item=V> (both where V: ToSpannerValue) if possible

[pjenvey]

Maybe less than an 8 score technically but there's some uncertainty on how this will all work