Implement `vector` methods for `DataList`

[ianfr]

It would be really good to create methods for DataList that act in a way very similar to std::vector such as:

• at()
• push_back()
• begin()
• end()
• size()

Along with a larger selection of constructors - maybe even a curly brace initializer!

This would let users use DataLists as a direct stand-in for vectors plus all the ADSL functionality.

These methods can probably just be wrappers around the DataList.vals field's methods.