allow warp::path!() call with no arguments

[utkarshkukreti]

This is a feature request + implementation for it. Let me know if you think this is a bad idea.

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It is equivalent to warp::path::end(). This is useful to have some symmetry
when defining multiple routes with path!:

warp::path!().map(|| 0)
.or(warp::path!("add" / u32 / u32).map(|a, b| a + b))
.or(warp::path!("mul" / u32 / u32).map(|a, b| a * b))

[utkarshkukreti]

Rebased!

[masklinn]

FWIW I had this exact same need / request recently. I replaced the not-working path!() call by an end() but it feels unclear and icky.

[jxs]

thanks!