Refactor: moves memrchr to global impl for potential future use

src/common/impl/memrchr.c

@@ -0,0 +1,19 @@
+#include "common/memrchr.h"
+#include <stddef.h>
+#include <stdint.h>
+
+void* memrchr(const void* s, int c, size_t n)
+{
+    if (n == 0) return NULL;
+
+    const uint8_t uc = (uint8_t) c;
+
+    const uint8_t* p = (const uint8_t*) s + n;
+
+    while (n--)
+    {
+        if (*--p == uc) return (void*) p;
+    }
+
+    return NULL;
+}