Our existing implementation of stream piping was extremely ad-hoc. It
did nothing to handle closed/errored streams, and did not read from or
write to streams in a way required by the spec.
This new implementation uses a custom JS::Cell to drive the read/write
loop.
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
