Macro fmtln

macro_rules! fmtln {
    ($fmt:ident, $fmtstring:expr, $($fmtargs:expr),*) => { ... };
    ($fmt:ident, $arg:expr) => { ... };
    ($_:tt, $($args:expr),+) => { ... };
    ($_:tt) => { ... };
}

A macro that simplifies the usage of the Formatter by allowing format strings.