Add rustversion::cfg! macro

[dtolnay]

A function-like procedural macro named cfg! was disallowed prior to Rust 1.38:

error: name `cfg` is reserved in macro namespace
   --> src/lib.rs:233:8
    |
233 | pub fn cfg(input: TokenStream) -> TokenStream {
    |        ^^^

but this restriction was lifted by rust-lang/rust#62243 (rust-lang/rust@3274507) and rust-lang/rust#62476.

Usage:

if rustversion::cfg!(nightly) {
    ...
} else {
    ...
}

Closes #7.