Re-implement the flat format ourselves

[ana-pantilie]

As a Plutus Core developer, I want to have an implementation of the binary format that we control, so that I can be confident it won’t change due to upstream changes, and so that we can change it ourselves easily.

AC:

• The new implementation is backwards compatible and can deserialize all old scripts.

[effectfully]

That's a great issue... Why reimplement anything though? Can't we just literally fork the repo and be done with it?

[Unisay]

I'd stick with the current flat implementation for as long as it remains backwards compatible, reserving a fork- option for a hypothetical situation when flat introduces a change we don't want to incorporate.