Figure out how to recycle it's in the REPL

[rahulmutt]

Description

The Eta REPL now generates a unique it[n] for every expression that is evaluated in the REPL, and there is currently no way to tell the REPL to release an existing it[n] from the heap.

@paulp suggested using SoftReference's but that would mean that a long running REPL session would silently drop heap references in eta-serv without the compiler knowing about it which would lead to unexpected exceptions if the GC'd reference is used again. We would need to think carefully about the consequences before implementing a solution for this.

In general, how do we know that a user no longer needs an expression in the REPL? Maybe it's just a matter of giving the user a REPL command like :kill [n] that will kill an it[n]?