[General] Move general traits in sceleton crate

[mhatzl]

Moving traits like Render into a sceleton crate that core and inline may depend on would allow us to have render implementations
already in the inline crate for inline elements.

In addition, feature flags could be used to define the output formats that should be included.
This could help to make the inline crate even more indipendent from all other crates and be more usable as standalone.