Sometimes, methods have different purity than corresponding functions

[catamorphism]

We ran into a problem where option::get is pure, but if you write foo.get(), it's considered impure, because the iface declaration doesn't mark anything "pure". There's a whole bunch of inconsistencies like this in option. More generally, a really simple form of purity inference would address this without having to do a bunch of tedious work.

[brson]

Often the reason for this is the restriction on self type argument modes.

[brson]

Where possible we should just delete the functions in favor of methods imo.

[bblum]

deleting functions in favour of methods would leave impure ones only.

[thestinger]

pure is now fully removed from the language, closing this since #3080 is already open about unsafe on methods.