[Mammoth] Move thread and process to the proc/ folder

lib/mammoth/proc/thread.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <glacier/status/error.h>
+#include <stdint.h>
+
+class Thread {
+ public:
+  typedef void (*Entry)(void*);
+
+  Thread() : thread_cap_(0) {}
+  Thread(Entry e, const void* arg1);
+
+  [[nodiscard]] glcr::ErrorCode Join();
+
+ private:
+  uint64_t thread_cap_;
+};