surpress warnings when specifying `--loop` and `--loop --count`

[h-mikisato]

When specifying --loop and --loop --count options, some compile warning is shown while compile. This PR is for surpressing them.

Target warnings are:

• "Unused imports" when not using std::io::prelude in script
• "Unnecessary braces around function argument" in some cases
  • ex. sample command in rust-script.org $ cat now.ers | rust-script --count --loop "|l,n| println!(\"{:>6}: {}\", n, l.trim_right())"
• "Trait objects without an explicit dyn" at output_any variable
• "Variable does not need to be mutable" at stdin variable

These are tested in rustc 1.57.0.

Thank you.

[fornwall]

Thanks! This is now published in version 0.20.0.