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!

can you please add the attribute "impure" to this function to indicate to the java backend that this function accesses state outside its inputs?