There is no need to have `RSAPrivateKey`, `RSAPublicKey`, `ECPrivateKey`
and `ECPublicKey` to be templatize to utilize different implementation
of numbers.
Previously, `ECDH::generate_key` was implemented by storing a
`ByteBuffer` in the `InternalKeyData`. This improves the implementation
by using internal structures of already-parsed data.
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
