Implement mach headers

Note: this crate exposes the user space Mach API, that is, the one that gets linked via `libSystem` and/or `libsystem_kernel`. It's headers are in `/usr/include/mach`.

This approximately summarizes the current implementation:

- [x] `boolean.h`
- [x] `bootstrap.h`
- [x] (maybe incomplete) `clock.h`
- [x] (maybe incomplete) `clock_priv.h`
- [x] (maybe incomplete) `clock_reply.h`
- [x] (maybe incomplete) `clock_types.h`
- [x] (maybe incomplete) `dyld_kernel.h`
- [x] (maybe incomplete) `error.h`
- [x] (maybe incomplete) `exc.h`
- [ ] `exception.h`
- [x] (maybe incomplete) `exception_types.h`
- [ ] `host_info.h`
- [ ] `host_notify.h`
- [ ] `host_priv.h`
- [ ] `host_reboot.h`
- [ ] `host_security.h`
- [ ] `host_special_ports.h`
- [x] (maybe incomplete) `kern_return.h`
- [ ] `kmod.h`
- [ ] `lock_set.h`
- [ ] `mach.h`
- [ ] `mach_error.h`
- [ ] `mach_host.h`
- [ ] `mach_init.h`
- [ ] `mach_interface.h`
- [ ] `mach_param.h`
- [x] (maybe incomplete) `mach_port.h`
- [ ] `mach_syscalls.h`
- [ ] `mach_time.h`
- [x] (maybe incomplete) `mach_traps.h`
- [x] (maybe incomplete) `mach_types.h`
- [ ] `mach_vm.h`
- [ ] `mach_voucher.h`
- [ ] `mach_voucher_types.h`
- [ ] `machine.h`
- [x] (maybe incomplete) `memory_object_types.h`
- [x] (maybe incomplete) `message.h`
- [ ] `mig.h`
- [ ] `mig_errors.h`
- [ ] `mig_strncpy_zerofill_support.h`
- [ ] `mig_voucher_support.h`
- [ ] `ndr.h`
- [ ] `notify.h`
- [ ] `policy.h`
- [x] (maybe incomplete) `port.h`
- [ ] `port_obj.h`
- [ ] `processor.h`
- [ ] `processor_info.h`
- [ ] `processor_set.h`
- [ ] `rpc.h`
- [ ] `sdt.h`
- [ ] `semaphore.h`
- [ ] `shared_memory_server.h`
- [ ] `shared_region.h`
- [ ] `std_types.h`
- [ ] `sync.h`
- [ ] `sync_policy.h`
- [x] (maybe incomplete) `task.h`
- [x] (maybe incomplete) `task_info.h`
- [ ] `task_inspect.h`
- [ ] `task_policy.h`
- [ ] `task_special_ports.h`
- [x] (maybe incomplete) `thread_act.h`
- [ ] `thread_info.h`
- [ ] `thread_policy.h`
- [ ] `thread_special_ports.h`
- [ ] `thread_state.h`
- [x] (maybe incomplete) `thread_status.h`
- [ ] `thread_switch.h`
- [ ] `time_value.h`
- [x] (maybe incomplete) `vm_attributes.h`
- [x] (maybe incomplete) `vm_behavior.h`
- [x] (maybe incomplete) `vm_inherit.h`
- [ ] `vm_map.h`
- [x] (maybe incomplete) `vm_page_size.h`
- [x] (maybe incomplete) `vm_param.h`
- [x] (maybe incomplete) `vm_prot.h`
- [x] (maybe incomplete) `vm_purgable.h`
- [x] (maybe incomplete) `vm_region.h`
- [x] (maybe incomplete) `vm_statistics.h`
- [x] (maybe incomplete) `vm_sync.h`
- [ ] `vm_task.h`
- [x] (maybe incomplete) `vm_types.h`