Problem:    NFA regexp uses tolower() to compare ignore-case. (Thayne McCombs)
Solution:   Use utf_fold() when possible. (ref. neovim #12456)

Problem:    Many type casts are used for vim_strnsave().
Solution:   Make the length argument size_t instead of int. (Ken Takata,
            closes #5633)  Remove some type casts.

Problem:    ml_delete() often called with FALSE argument.
Solution:   Use ml_delete_flags(x, ML_DEL_MESSAGE) when argument is TRUE.

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().

Problem:    Using the same loop in many places.
Solution:   Define more FOR_ALL macros. (Yegappan Lakshmanan, closes #5339)

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.

Problem:    :lockmarks does not work for '[ and '].
Solution:   save and restore '[ and '] marks. (James McCoy, closes #5222)

Problem:    After :diffsplit closing the window does not disable diff.
Solution:   Add "closeoff" to 'diffopt' and add it to the default.

Problem:    Typos in comments.
Solution:   Fix the typos. (Dominique Pelle, closes #5160)  Also adjust
            formatting a bit.

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.