Adding input macro

[Cr0a3]

I created an input macro with which you can easly get user input.
Example:

pub fn main() {
    let value: String = input!("enter value: ");
    println!("your value: {}", value);
 }

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @cuviper (or someone else) some time within the next two weeks.

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[Noratrieb]

Thank for the PR! This has been proposed many times, for example rust-lang/rfcs#3183 and rust-lang/rfcs#3196 and #75435 and probably more discussion on internals.rust-lang.org. You should probably follow that discussion instead and get involved there if you have good arguments for this that haven't already been brought up.

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