Add type attributes (and wire them through name mangling) for sealed types

[davidchisnall]

Currently, we use void* for all sealed types. We should be able to write [[cheriot::sealed]] struct T * and propagate the type through the rest of the system.

All operations on sealed types other than an explicit cast should fail. The token_unseal function will take a [[cheriot::sealed]] void* and return a void*, but it is written in assembly.