The main streams AO file has gotten very large, and is a bit difficult
to navigate. In an effort to improve DX, this migrates ReadableStream
AOs to their own file. And the helper classes used for the tee and pipe-
to operations are also in their own files.
This will be needed by Streams. To support this, we now store callbacks
in a hash table, keyed by an ID. Callers may use that ID to remove the
callback at a later point.
This is very clearly a very dangerous API to have, and was causing
a crash on Linux as a result of a stack use-after-free when visiting
https://www.index.hr/.
Fixes#3901
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
