Generate exact JS buffer types for exact IDL buffer arguments instead
of widening them to BufferSource or ArrayBufferView.
This fixes cases like TextEncoder.encodeInto(), whose IDL requires
a Uint8Array destination. Previously the generated binding accepted
any BufferSource, so DataView, other typed arrays, and
ArrayBuffer-backed values were let through. With exact conversion,
those are rejected at the binding layer as expected.
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