Change 'RawPtr' to 'PtrExt' due to updated API in Rust master.
Change 'RawPtr' to 'PtrExt' due to updated API in Rust master.

placrosse/capnproto-rust,dwrensha/capnproto-rust,larquin/capnproto-rust,danburkert/capnproto-rust,maurer/capnproto-rust,tempbottle/capnproto-rust