Problem: printf() gives an error when the argument for %s is not a string.
(Ozaki Kiichi)
Solution: Behave like invoking string() on the argument. (Ken Takata)
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: 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: Getting an item from a NULL dict crashes. Setting a register to a
NULL list crashes. (Nikolai Pavlov, issue #768) Comparing a NULL
dict with a NULL dict fails.
Solution: Properly check for NULL.
Problem: Functions specifically for testing do not sort together.
Solution: Rename garbagecollect_for_testing() to test_garbagecollect_now().
Add test_null_list(), test_null_dict(), etc.
Problem: Comparing a function that exists on two dicts is not backwards
compatible. (Thinca)
Solution: Only compare the function, not what the partial adds.
