Break up headers into constituent parts

This is part of a broader change to make header files better represent the
dependencies between one another (see
https://github.com/jemalloc/jemalloc/issues/533). It breaks up component headers
into smaller parts that can be made to have a simpler dependency graph.

For the autogenerated headers (smoothstep.h and size_classes.h), no splitting
was necessary, so I didn't add support to emit multiple headers.

include/jemalloc/internal/mutex_structs.h

@@ -0,0 +1,24 @@
+#ifndef JEMALLOC_INTERNAL_MUTEX_STRUCTS_H
+#define JEMALLOC_INTERNAL_MUTEX_STRUCTS_H
+
+struct malloc_mutex_s {
+#ifdef _WIN32
+#  if _WIN32_WINNT >= 0x0600
+	SRWLOCK         	lock;
+#  else
+	CRITICAL_SECTION	lock;
+#  endif
+#elif (defined(JEMALLOC_OS_UNFAIR_LOCK))
+	os_unfair_lock		lock;
+#elif (defined(JEMALLOC_OSSPIN))
+	OSSpinLock		lock;
+#elif (defined(JEMALLOC_MUTEX_INIT_CB))
+	pthread_mutex_t		lock;
+	malloc_mutex_t		*postponed_next;
+#else
+	pthread_mutex_t		lock;
+#endif
+	witness_t		witness;
+};
+
+#endif /* JEMALLOC_INTERNAL_MUTEX_STRUCTS_H */