ENC/DEC: implement compression via lz4

This speeds up encryption/decryption considerably, but also
causes complete failure for any non-recoverable char in a message,
since the decompression will fail then.

Also remove the double base64 encode/decode.

README.md

@@ -9,6 +9,7 @@ implementation with the primary goal of learning.
 
 * FLINT (compiled with gmp and mpfr)
 * glib-2.0
+* lz4 (https://code.google.com/p/lz4)
 * pkgconfig (for the build only)
 
 ### Compiling the library