[5.0.0 Breaking change] Using user defined value type for timers

[Amxx]

The structures used in the timers library prevent storage packing (in particular, this affects Governor's Proposals).

This has to effect that makes it a breaking change:

• Storage incompatibility (data is now packed) in the Governor contract.
• New timers being "value type", makes it impossible to write "setDeadline" and "reset" methods.

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

This needs to be rebased on next-5.0.

[frangio]

@Amxx Do we still want to do this or is your current belief that it's better to use uint48 types instead of introducing the timer abstraction?

[Amxx]

In 5.0 we definitelly want to move away from struct-based timers has this prevents proper packing.

This leaves us with some options:

• using UDVT (as shown in the PR)
  • the underlying type should probably be changed from uint64 to uint48
  • we'll continue to have both timestamp & blocknumber
• remove the Timers library altogether

A third option might be to add an abstract "withClock" contract, that implement EIP-5805's clock(), a UDVT, and the internal function to compare that UDVT to the clock
(I'm not sure about this third option, need more thoughts)

[frangio]

Closing due to deprecatioon (#4062).