Remove proc-macro2 from default features

[LukasKalbertodt]

Default features in library crates that are not used by the vast majority of downstream crates, should likely not be default features. A quick GitHub search shows: there are 46 total repositories with litrs in Cargo.toml and 35 of those also have proc-macro2 in Cargo.toml. And 5 repositories set default-features = false for litrs. This shows what is well known: people forget to disable default features they don't need. That leads to more dependencies that are compiled for no reason. Though 35 of 46 sounds like most people use the proc-macro2 feature. Well, I have looked through a few of those repos and several do use proc-macro2 in general, but not for litrs.

So I'm inclined to make proc-macro2 a non-default feature of litrs.