Add base64 support

Lua Server Pages may now call the EncodeBase64() and DecodeBase64()
functions should they wish to do things like emit embeded data URIs

See #97

libc/str/a64l.c

@@ -38,7 +38,9 @@ static const signed char kBase64i[256] = {
 };
 
 /**
- * Converts base64 to 32-bit integer.
+ * Converts base64 to 32-bit integer, the posix way.
+ * @see l64a() for inverse
+ * @see DecodeBase64()
  */
 long a64l(const char *s) {
   uint32_t i, v, x;