Problem:    Can modify a:000 when using a reference.
Solution:   Make check for locked variable stricter. (Ozaki Kiichi,
            closes vim/vim#3930)
https://github.com/vim/vim/commit/05c00c038bc16e862e17f9e5c8d5a72af6cf7788

Problem:    Cannot use a lambda with quickfix functions.
Solution:   Add support for lambda. (Yegappan Lakshmanan, closes vim/vim#6499)
https://github.com/vim/vim/commit/d43906d2e5969288f239df851f5ad7b1dc2c7251

Problem:    Code to handle callbacks is duplicated.
Solution:   Add callback_T and functions to deal with it.
https://github.com/vim/vim/commit/3a97bb3f0f8bd118ae23f1c97e55d84ff42eef20

Port Vim's put_callback() as callback_put()
because Neovim's naming convention is {type}_{action},
not {action}_{type}.

Renaming put_callback type as PutCallback.
https://neovim.io/develop/style-guide.xml#Type_Names

Problem:    Cannot use "z=" when 'spell' is off.
Solution:   Make "z=" work even when 'spell' is off. (Christian Brabandt,
            Gary Johnson, closes vim/vim#6227)
https://github.com/vim/vim/commit/152e79e94bb935e75b866bd55479648cde11066a

Resolve pvs/v560 error.
mods is nonnull in the modified inner if-condition.
because it is checked for NULL in the outer if-condition.

vim_strsave() returns nonnull.

Windows handles environment variable names case-insensitively (i.e.,
%Path% can be accessed via %PATH%).  Since nvim explicitly creates the
environment block for jobs, we need to ensure it doesn't contain
duplicate names with different cases (e.g., both %Path% and %PATH%) as
that can confuse processes using the environment.

`f_environ()` now converts all env var names to upper-case on Windows,
since it does duplicate detection and also provides the base environment
for jobs.  Similarly, `create_environment()` converts any env var names
supplied in the job opts to upper-case so they can be compared against
what we get from `f_environ()`.

Closes #14541

Problem:    Cannot intercept a yank command.
Solution:   Add the TextYankPost autocommand event. (Philippe Vaucher et al.,
            closes vim/vim#2333)
https://github.com/vim/vim/commit/7e1652c63c96585b9e2235c195a3c322b1f11595

Problem:    Line() only works for the current window.
Solution:   Add an optional argument for the window to use.
https://github.com/vim/vim/commit/8e0a8e7eb7c177807f44db6b76d8e52314248ab5