TransactionView: read_entry

[apfitzge]

Problem

• TransactionView design was mainly towards block production but doesn't super easily work with Entry in block validation

Summary of Changes

• Write a function to a serialized entry into header + TransactionView

Future

Brief description of the approach in block-validation:

• serialized entry data must be wrapped in an Arc .
• read header information (this copies, since afaict rust has no way for me to get a nicely aligned Vec)
• return iterator over Result. each view has a clone of the arc data + the offset that specific transaction starts at
  • this nicely does not force us to allocate a Vec on consuming side
• in block validation we must check that all results are Ok for the Entry to be considered Ok

Fixes #