Macro host_api

macro_rules! host_api {
    (($env:ident: Host<$E:ty>; $($name:ident: $type:ty),*) $body:block) => { ... };
    (($env:ident: Host<$E:ty>; $($name:ident: $type:ty),*) -> $rt:ty $body:block) => { ... };
}