Rework scope collection serialization

[gwen-lg]

Draft as the code need check and clean. But advanced enough to have feedback on the intention.

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Add a specific (de)serialization of ScopeCollection (independent of FrameData serialization).
This is done to avoid but and improve readability of the code.
This simplify the code by avoid mixed usage of serialized scopes of FrameData for scope_collection or delta_scope.

• bump puffin file magic to b"PUF1".
• increment puffin_http protocol version to 3.
• add scopes_collection data header : b"PSC1".

Related Issues

• Saving as .puffin _sometimes_ breaks #248
• Wrong scope descriptions in puffin viewer when running profiled program multiple times #200

[gwen-lg]

@emilk : not sure if you see this, as it's a draft.