Adds support for List and Map `DynVal`.

[angerman]

In aristidb/aws/pull/177, @puffnfresh added support for

• Null
• Lists (L)
• Map (M)

where Lists are represented as DList (Vector DValue) and Maps as DMap (Map Text Dvalue).

This adds DynVal instances for Vector and Map Text to allow them to be used when parsing a structure into records.

[angerman]

This replaces aristidb/aws/pull/192, fixing up the commit messages.

[aristidb]

Can you please add short comments to the code explaining the difference between the [], Set, Vector and Map instances? Because I do find that somewhat confusing.

(Of course, partially the confusingness stems from the fact that Amazon first tried to use a simplistic data model and then added more complex features, while leaving the simplistic form distinct and present.)