[vm/ffi] Make Pointer and DynamicLibrary non-special classes

[dcharkes]

As discussed with @sjindel-google, we should probably make Pointer a normal (not VM special) class. This doesn't change the public API of the FFI in any way, but simplifies its implementation.

Historically Pointer was implemented this way as it was modeled after TypedData, but we only the actual integer address to do special things in the VM (when loading and storing data and when doing foreign calls). Loading the address from the Pointer (or storing it in a new Pointer) does not need to be special in any way.

Edit: idem for DynamicLibrary.

[dcharkes]

This would enable us to declare nullptr as const and make the fromAddress constructor be used in const expressions.

[dcharkes]

This is no longer a goal, because we will share some low level logic between TypedData and Pointer. #40767