Equivalent of `cfg_select`?

There are some contexts where the grammar of rust allows one macro, but not arbitrary expressions, e.g. attributes on expressions and impl heads.

Is there an interest in a `cfg_select!` macro that does the equivalent of `std::cfg_select!` for rust versions?

```rust
rustversion::cfg_select! {
nightly => { Vec },
_ => { Vec },
}
// expands on nightly to
// Vec
// on stable to
// Vec
```

Note in the example above that attributes are [not allowed on generic arguments](https://doc.rust-lang.org/reference/grammar.html#grammar-summary-GenericArgList), hence naming the full type. A type alias is also not a good option since rust checks that all parameters are used and errors when the optional allocator argument is unused.

Would be interested in this and follow up with a PR if you are too.