Use ?-deps for lexical-core's metafeatures

[Manishearth]

This crate reexports functionality from various subcrates. Unfortunately, it also has some metafeatures that attempt to enable functionality across these crates. Due to how Cargo feature dependencies work, enabling a feature that enables lexical-write-integer/foo will automatically enable lexical-write-integer as a dependency.

The ? syntax fixes this.

Would be nice to get a release with this too.

[Manishearth]

r? @myrrlyn