[Mammoth] Move most classes to the mmth namespace.

lib/mammoth/proc/process.h

@@ -4,5 +4,9 @@
 #include <stdint.h>
 #include <ztypes.h>
 
+namespace mmth {
+
 glcr::ErrorCode SpawnProcessFromElfRegion(uint64_t program,
                                           z_cap_t yellowstone_client);
+
+}  // namespace mmth