Macro persistentcache::cache_func

macro_rules! cache_func {
    (Redis, $host:expr, fn $f:ident($($x:ident : $t:ty),*) -> $r:ty $b:block) => { ... };
    (File, $dir:expr, fn $f:ident($($x:ident : $t:ty),*) -> $r:ty $b:block) => { ... };
    (Redis, $host:expr, $prefix:expr, fn $f:ident($($x:ident : $t:ty),*) -> $r:ty $b:block) => { ... };
    (File, $dir:expr, $prefix:expr, fn $f:ident($($x:ident : $t:ty),*) -> $r:ty $b:block) => { ... };
    ($f:ident($($x:ident),*), $b:block, $prefix:expr) => { ... };
}

Cache an entire function.