Redesign Transaction V5 serialization

[teor2345]

Motivation

It's easy to accidentally serialize V5 transaction fields in the wrong order. So we should go back to the design stage for V5 transaction serialization.

Solution

• Update the v5 transaction RFC with an explicit design for serialization
  • Only have serialize and deserialize impls on types that are serialized as continuous bytes in V4 and V5 transactions
  • To avoid accidental serialization of Outputs in the V4 format in V5 transactions, add a wrapper type, and move the serialization impls to that type
  • List the types that have serialization impls, noting new impls
  • Explain how to serialize types that don't have serialization impls, by splitting those types up into their parts
• Implement the design for sapling spends and outputs, to make sure it works
• Security: Implement trusted vector preallocation for spends and outputs

The code in this pull request has:

• Documentation Comments
• Property Tests

Review

@oxarbitrage should review, because it blocks ZcashFoundation#1996

@dconnolly might also want to have a look, because it's a redesign.

Follow Up Work

Add the serialization impls to the implementation section of ZcashFoundation#1829 and ZcashFoundation#1979 (done!)

[oxarbitrage]

I think in general it looks good, i am going to merge here and we can discuss further in the zebra PR.