style: remove extra const keyword from interfaces

Const variables in prototypes don't add much value, but make the code
larger and redundant. Remove these const keywords.
Note, this doesn't apply to pointers to const memory.

tests/test.c

@@ -16,9 +16,9 @@
 
 static uint8_t char_to_bin(char c)
 {
-    return c <= '9'
-               ? (uint8_t)(c - '0')
-               : c <= 'Z' ? (uint8_t)(c - 'A' + 10) : (uint8_t)(c - 'a' + 10);
+    return c <= '9'   ? (uint8_t)(c - '0')
+           : c <= 'Z' ? (uint8_t)(c - 'A' + 10)
+                      : (uint8_t)(c - 'a' + 10);
 }
 
 void test_hex_to_bin(const char *hex, uint8_t *bin, size_t *bin_len)