Support Go garbage-collected ciphertexts (ethereum#34)

Optimize cgo <-> tfhe-rs performance by:
 * not deserializing the server key on every operation
 * not serializing/deserializing the ciphertexts on every operation
 * making sure a ciphertext is serialized/deserialized at most once
 * not copying bytes when passing them from Go to C
 * caching cihpertext serialization and hash for later uses

Keep pointers to C-allocated ciphertexts in the `verifiedCiphertexts`
EVM variable instead of their serialization. Hook these C-allocated
ciphertexts to the gargage collector by utilizing `setFinalizer()`.

Run the GC based periodically, based on number of allocated
ciphertexts.

Add an unit test for `tfhe.go`.

.gitignore

@@ -49,5 +49,5 @@ profile.cov
 **/yarn-error.log
 
 # TFHE
-core/vm/tfhe*
+core/vm/tfhe.h
 core/vm/lib/libtfhe*