Problem: Error when defining a Lambda with index of a function result.
Solution: When not evaluating an expression and skipping a function call,
set the return value to VAR_UNKNOWN.
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: filter() and map() either require a string or defining a function.
Solution: Support lambda, a short way to define a function that evaluates an
expression. (Yasuhiro Matsumoto, Ken Takata)
