Remove more nonstandard stuff from cosmopolitan.h

Fixes #61

libc/alg/reverse.internal.h

@@ -0,0 +1,31 @@
+#ifndef COSMOPOLITAN_LIBC_ALG_REVERSE_H_
+#define COSMOPOLITAN_LIBC_ALG_REVERSE_H_
+#include "libc/bits/xchg.internal.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+
+/**
+ * Reverses array.
+ *
+ * @param ARRAY is a typed array or a pointer to one
+ * @param COUNT is the number of items
+ * @return pointer to start of array
+ * @see ARRAYLEN()
+ */
+#define reverse(ARRAY, COUNT)              \
+  ({                                       \
+    autotype(&(ARRAY)[0]) Array = (ARRAY); \
+    size_t Count = (COUNT);                \
+    if (Count) {                           \
+      size_t Start = 0;                    \
+      size_t End = Count - 1;              \
+      while (Start < End) {                \
+        xchg(&Array[Start], &Array[End]);  \
+        ++Start;                           \
+        --End;                             \
+      }                                    \
+    }                                      \
+    Array;                                 \
+  })
+
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_ALG_REVERSE_H_ */