Remove proc-macro renames in Jan 2021

[Peternator7]

The Proc-Macro renames feature (in which you can enable the feature and the proc-macro becomes Strum<MacroName>) increases the surface area of things that need to be tested pretty substantially, but provides much less benefit in rust 2018 since macros can be scoped and imported just like any other feature.

Given Rust 2018 has been out for almost 2 years now, the first version of strum in 2021 will remove them and using import paths will be the only supported way to disambiguate proc-macros between crates.

See this wiki page for more context about the original feature. https://github.com/Peternator7/strum/wiki/Macro-Renames