Macro parallel

macro_rules! parallel {
    (impl $fblock:block [$($c:expr,)*] [$block:expr $(, $rest:expr)*]) => { ... };
    (impl $fblock:block [$($blocks:expr,)*] []) => { ... };
    ($fblock:block, $($blocks:block),*) => { ... };
}

Runs a list of blocks in parallel. The first block is executed immediately on the current thread. Use that for the longest running block.