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:    Coverity reports bogus NULL check.
Solution:   When checking for a variable in the funccal scope don't pass the
            varname.

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:    No error for using function() badly.
Solution:   Check for passing wrong function name. (Ken Takata)

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:    Closures are not supported.
Solution:   Capture variables in lambdas from the outer scope. (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.

Problem:    eval.c is too big.
Solution:   Move user functions to userfunc.c