Add an explicit recur special

[obriencj]

Currently, with tail-call-recursion enabled, a function will check if a call in a tail position is invoking itself. This relies on functions having a self-reference, which is provided by a closure generated around all functions with a name of "", to which the invocation can do an identity test.

Recursion in this fashion relies on the function having a name as well.

Add a form named recur which will explicitly act to restart the function.

Recur should raise a compile-time exception if used in a non-tail-call position.