Make SlashingSpans Public (#6961)

* Make SlashingSpans Public

Offchain Applications will often need to inspect this type because it is directly used in staking election, thus worthy of being `pub`. Rest of the slashing api can remain private, only this and the `fn last_non_zero_slash()` of `SlashingSpans` are of interest.

* Update frame/staking/src/lib.rs