Extract the block fitting logic from the block assembler logic

Summary:
This will make it easy to check if a tx would fit a block without having to go through the expensive block construction process.

This is a refactor only, there is no change in behavior.

Test Plan:

ninja all check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17793