It currently has the syntax
`current_rustc_version!(env!("CFG_RELEASE"))` where the
`env!("CFG_RELEASE")` part looks like a normal expression but it is
actually parsed and processed by the `current_rustc_version` macro.
The documented rationale for this is that you'll find it if you grep for
`env!("CFG_RELEASE")`. But I think that's of very little use -- I would
personally grep for just "CFG_RELEASE" -- and it complicates the macro,
requiring the use of `syn`.
This commit simplifies the macro.
