This patch introduces a new data structure, that allows generic
structures to be interned. This works similarly to how normal
string interning works, however it is specialized to the
thread model for WR (explained at the top of intern.rs).

The effect of this change is that clip nodes are de-duplicated
and persisted between both frames and display lists.

This has three primary benefits:
 * Since they are de-duplicated, the handle for an interned
   structure uniquely identifies it. This is very useful for
   future use where we want to be able to quickly and cheaply
   compare if the contents of a cached picture matches that
   of a new display list.
 * Since they are persisted between display lists, the GPU
   cache handles for the nodes remain valid. This means far
   fewer GPU cache update patches for types that are interned.
 * Since they are de-duplicated by content hash value, there
   are fewer clips overall used by the frame builder.

The plan in the future is to extend this to other primitive types,
as well as gradient stops, text runs etc. This will allow us to
very quickly check if a cached picture remains valid, even in the
presence of a completely new display list.

This adds a small amount of overhead to the scene builder thread,
(extra hashing) but reduces the CPU time in the render backend and
compositor threads, which is also a good tradeoff.