Adding #time function

#time should return the number of milliseconds since the epoch. However,
the return type is long, and long isn't supported in K as far as I know.
I made this commit, even though it's a mess, to get feedback from the
developers on how to properly do this. I was following [this pull
request](https://github.com/kframework/k/pull/967/files) which is why I
put it under IO but then I changed my mind in hooks.properties and made
time.k!