Constime

This is a tiny alternative to https://github.com/nhynes/comptime-rs, with no dependencies and <70 LOC.

Note that it currently does not support externs.

```rust fn main() { use constime::comptime;

println!(
    "Compiled {} seconds after unix epoch",
    comptime! {
        std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .expect("Time went backwards")
            .as_secs()
    }
);

} ```