Add an utility function to get the first timestamp of a slot

[LGLO]

Description

Add starting_timestamp function for Slot type.

Integration

This is an addition of public function to a type, so integration should be seamless for idiomatic use of Rust.

Review Notes

Since Slot is just a slot number, the it's starting timestamp depends on SlotDuration which is a parameter to the added function. This function can be seen as dual to existing fn from_timestamp.
Because there is a potential for overflow, the return type is Option.

Q1: should I introduce tests for in this crate and add cases for both case: overflow (None) and no overflow (Some)?

Q2: How can I add labels? IMO they should be T0-node and D0-easy but I cannot add them using GH interface.

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
• My PR follows the labeling requirements of this project (at minimum one label for T
  required)
  • External contributors: ask maintainers to put the right label on your PR.
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[davxy]

Suggested change

	/// The first timestamp of the slot. Returns None if would overflow for given SlotDuration.
	pub fn first_timestamp(&self, slot_duration: SlotDuration) -> Option<Timestamp> {
	/// The timestamp of the slot given the duration.
	///
	/// Returns `None` if would overflow for given `SlotDuration`.
	pub fn to_timestamp(self, slot_duration: SlotDuration) -> Option<Timestamp> {

I'd rather change the api to this (Slot is Copy)

[bkchr]

Or maybe just call it timestamp. Not really required to have any longer name.

[LGLO]

I've renamed to timestamp, but kept "first" in the documentation, because it returns the first timestamp of the slot

[github-actions]

Review required! Latest push from author must always be reviewed

[bkchr]

@LGLO can you please merge master again?

[LGLO]

@bkchr done