Use 'proc-macro = true' instead of crate-type

[ExpHP]

See https://github.com/rust-lang/cargo/issues/5310

Both ways of writing this appear to work when compiling plex itself; however, crate-type = ["proc-macro"] breaks rustdoc on downstream crates. proc-macro = true is the syntax used in all documentation I can find of the feature:

• https://doc.rust-lang.org/book/first-edition/procedural-macros.html
• https://doc.rust-lang.org/unstable-book/language-features/proc-macro.html

[goffrie]

Thanks!