Sized method
Allocates (or reuses) a slot of length elements and returns its pointer,
without writing any data into it.
Unlike Array or Fill, the contents are left uninitialized, useful when
the buffer will be populated by a C call rather than from Dart.
key defaults to 'Sized<C>'.
Implementation
P Sized(int length, {String? key}) => At(key ?? 'Sized$X', length);