Rename Burn to Slash in Assets

[joepetrowski]

Renames burn to slash to update API for Statemint. Also removes duplicate Burned and Issued events.

[apopiak]

Should we also adjust the event to Slashed?

[joepetrowski]

Should we also adjust the event to Slashed?

I think it makes sense in the context of assets, but don't want to rename the helper functions (e.g. do_burn) as it creeps into the fungibles trait.

[Swader]

What is the rationale behind this change?

[apopiak]

@gavofyork Can you give your reasoning?

[Swader]

Still interested in learning more about the reasoning here. To me, slash has very clear connotations of "owner of asset having promised to do something and failing to deliver on that promise, thereby getting slashed". To me at least, slash clearly communicates punishment. Burn on the other hand covers both self-inflicted ops and burns from an admin, with or without a previous commitment (e.g. tether is ordered to burn some amount in my account, but also I can burn some amount of USDT in my account on my own - in neither case did I have a commitment to the network the dishonoring of which would punish me through a slash).