Problem: A funccal is garbage collected while it can still be used.
Solution: Set copyID in all referenced functions. Do not list lambda
functions with ":function".
Problem: Using function() with a name will find another function when it is
redefined.
Solution: Add funcref(). Refer to lambda using a partial. Fix several
reference counting issues.
Problem: User defined functions can't be a closure.
Solution: Add the "closure" argument. Allow using :unlet on a bound
variable. (Yasuhiro Matsumoto, Ken Takata)
Problem: Using submatch() in a lambda passed to substitute() is verbose.
Solution: Use a static list and pass it as an optional argument to the
function. Fix memory leak.
