Macro layout 

macro_rules! layout {
    ($name:ident u($bits:expr); $group:expr) => { ... };
    ($name:ident e($exponent:expr)m($mantissa:expr); $group:expr) => { ... };
    ($name:ident = $text:expr; [$group:expr] in $size:expr) => { ... };
    ($name:ident u($bits:expr)) => { ... };
    ($name:ident i($bits:expr)) => { ... };
    ($name:ident e($exponent:expr)m($mantissa:expr)) => { ... };
    ($name:ident; [$group:expr] in $size:expr) => { ... };
}

Defines an instance of DigitLayout.