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.

[resistor]

Some initial wiring up of the attribute here resistor@1c83b24

For some reason it seems to only work with old-style GNU attributes and not C++ attributes at the moment.

[resistor]

Reworked as a proper type annotation here: resistor@20a0771

[resistor]

Available to testing here: resistor@828e58a

[resistor]

Fix is in #88