Problem: The return value of mode() does not indicate that completion is
active in Replace and Insert mode. (Zhen-Huan (Kenny) Hu)
Solution: Add "c" or "x" for two kinds of completion. (Yegappan Lakshmanan,
closes#1397) Test some more modes.
Problem: When making a character lower case with tolower() changes the byte
cound, it is not made lower case.
Solution: Add strlow_save(). (Dominique Pelle, closes#1406)
Problem: str2nr() and str2float() do not always work with negative values.
Solution: Be more flexible about handling signs. (LemonBoy, closes#1332)
Add more tests.
