feat: add memo param to TxBuilder

[nocktoshi]

Adds ability to send a memo with the wasm builder.

Note: I still need to update iris UI to show the memo when signing. For now it works by blind signing.

Txn from Iris: https://nockblocks.com/tx/BkcNhQcSmRjQygQ4wYTLFh6WVKmSyNbKFCcKVyBYkZNUDzjwxUEfidh

Usage:

          const builder = new wasm.TxBuilder(feePerWord);
          const recipientDigest = new wasm.Digest(PAYMENT_ADDRESS);
          const refundDigest = new wasm.Digest(address);

          builder.simpleSpend(
            [note],
            [spendCondition],
            recipientDigest,
            amount,
            null,
            refundDigest,
            false,
            "My Memo" //New Memo Parameter
          );

[h33p]

Finally had time to look at this. If we are to support a standard for text encoding, it should be optimal. Few issues I can see:

• Not a fan of 1 byte per belt. We can encode arbitrarily long sequences as belts. See Base58Belts in iris-ztd/hash.rs for conversions. We would just an equivalent of to/from_bytes for Vec<Belt> holding type.
• There's no need to null terminate this sequence - it just ends

Both of these add up to transaction cost. If the following can be done to cut costs, they should be done. At that point, the standard can be adopted by the wallet officially.

[h33p]

@nocktoshi there is now efficient and compact atom<->belts conversion https://github.com/nockbox/iris-rs/blob/main/crates/iris-wasm/src/noun.rs#L48

[nocktoshi]

@h33p I have updated the pr to use the new atom<->belts conversion in noun.rs

How does it look?