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/l64a.c

@@ -19,7 +19,9 @@
 #include "libc/str/str.h"
 
 /**
- * Converts 32-bit integer to base64.
+ * Converts 32-bit integer to base64, the posix way.
+ * @see a64l() for inverse
+ * @see EncodeBase64()
  */
 char *l64a(long x) {
   static char b[7];