Add zcash_serialized_size() to ZcashSerialize trait

[oxarbitrage]

Motivation

I am not totally sure this is the way to do it but just a quick attempt to resolve #2778

I am open to just close the PR if this is not a good idea or make changes if needed.

Consensus Rules

The mempool of a node holds a set of transactions. Each transaction has a cost, which is an integer defined as:
max(serialized transaction size in bytes, 4000)

https://zips.z.cash/zip-0401#specification

Solution

Use the already in place zcash_serialize_to_vec() helper function to simplify the task of getting a serialized size of any object, in this case we are using it to get the size of a transaction.

Review

I think anyone can review.

[teor2345]

I think there's two ways we can be more efficient here:

• stop allocating RAM and writing data to it - just sum the serialized sizes instead
• calculate the serialized size once, then add it to UnminedTx as a new field

[teor2345]

Looks good, just a few performance tweaks we could do.

[teor2345]

Looks good!