Macro capsule::compose

macro_rules! compose {
    ($map:ident { $($key:expr => |$arg:tt| $body:block)+ }) => { ... };
    ($map:ident { $($key:expr => |$arg:tt| $body:block)+ _ => |$_arg:tt| $_body:block }) => { ... };
    ($map:ident { $($key:expr),+ => |$arg:tt| $body:block }) => { ... };
    ($map:ident { $($key:expr),+ => |$arg:tt| $body:block _ => |$_arg:tt| $_body:block }) => { ... };
    ($map:ident { _ => |$_arg:tt| $_body:block }) => { ... };
}

Composes the batch builders for the group_by combinator.