Add encode cache

[hwwhww]

What was wrong?

#36 part 1: Add cache for encode.

How was it fixed?

Basically copied the same logic from pyrlp.

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[jannikluhn]

Approved, even though I'm still not convinced that caching like this is very helpful. In practice, in which situations are we going to serialize the same object multiple times? I can only think of hashing, but if that's the only reason we could just cache the hash.

Another thought: We could set cached_ssz in ssz.decode as well, then we could avoid serialization in some cases completely (e.g. we get something from the network and send it to other peers).

[hwwhww]

Approved, even though I'm still not convinced that caching like this is very helpful. In practice, in which situations are we going to serialize the same object multiple times? I can only think of hashing, but if that's the only reason we could just cache the hash.

Yeah no good use case in my mind either in client side.

Another thought: We could set cached_ssz in ssz.decode as well, then we could avoid serialization in some cases completely (e.g. we get something from the network and send it to other peers).

Oh yes just noticed that there are also cache for decode: ethereum/pyrlp#95 (and https://github.com/ethereum/pyrlp/blob/7904316f3da420d8dda214dfe8fa168a8e866cfb/rlp/codec.py#L245-L258). Will take a look.