Sapling: add custom memosize

Do not review yet.

• Use fix !322 (merged)
• Add tests for !322 (merged)
• Add same tests for big_map
• Memo size is not fixed anymore and any size can be used up to 512 bytes using PUSH nat 42; SAPLING_EMPTY_STATE; ....

TODO:

• write a smart contract using a sapling state and accepting transaction with a different memo size.
• Accept { memo_size } as an argument to get a sapling state with the requested memo size.
• [ ] Rename SAPLING_EMPTY_STATE in EMPTY_SAPLING_STATE.
• PUSH nat 42; SAPLING_EMPTY_STATE --> SAPLING_EMPTY_STATE 42.
• remove default_memo_size everywhere.