Previously this was proxying the call through javascript, which lead to
unexpected crashes when functions returned things that js-api did not
like.
This commit also adds in the spec comments and fixes a few inaccuracies
that were present in the process.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
