Preparation to make decoder use less memory and API more type safe.
Now each scalar type has it's own struct type so it can store different
things and enables to have a scalar interface.
Also own types will enable experimenting with decode DLS designs like
using chained methods that are type aware.
json, yaml, toml, xml, html, csv are now normal formats and most of them also particiate
in probing (not html and csv).
Also fixes a bunch of bugs in to/fromxml, to/fromjq etc.
