threadshare: simplify Pad{Src,Sink} implementations
threadshare: simplify Pad{Src,Sink} implementations

Pad{Src,Sink}[Ref] delegate some functions to their respective
Pad{Src,Sink}Inner. Since they act as smart pointers, we can
safely implement the Deref trait to simplify the implementations.

sdroege/rsplugin,sdroege/rsplugin