Make function clones a lot cheaper
Use Rc to reduce the cost of function cloning.
In terms of performance, this is a 20% speed boost with this benchmark:
fn demo
echo a > /dev/null
echo a > /dev/null
echo a > /dev/null
echo a > /dev/null
echo a > /dev/null
echo a > /dev/null
echo a > /dev/null
echo a > /dev/null
end
for _ in 0..=1000000
demo
end