Expose user-friendly version of read_value! for users

[statiolake]

Sometimes we want to use the read value immediately, especially in very simple problem. For example it's useful if we can solve A + B problem like:

println!("{}", read!(i32) + read!(i32));

We have been using read_value! internally so all we need to do is to write a thin wrapper for that.
(Of course we can use read_value! itself even as it is now but it's hidden and a bit verbose for human writing.)