Problem:    Various functions not properly tested.
Solution:   Add more tests, especially for failures. (Yegappan Lakshmanan,
            closes vim/vim#5843)

https://github.com/vim/vim/commit/0e05de46226eb4e5ea580beefa71831f92d613d3

Cherry-pick test changes from patch 8.2.0427 and skip Test_has().
Cherry-pick Test_complete_wildmenu() change from patch 8.2.4339.

Prep for patch v8.2.0511.

Patch 8.1.0711 is too difficult to merge in 1 commit.

Problem:    get(func, dict, def) does not work properly.
Solution:   Handle NULL dict better. (Takuya Fujiwara, closes vim/vim#4734)
https://github.com/vim/vim/commit/f91aac5e3e3b8b1633d84eac2687ebbd76d8133b

Problem:    filter() and map() only accept a string argument.
Solution:   Implement using a Funcref argument (Yasuhiro Matsumoto, Ken
            Takata)

https://github.com/vim/vim/commit/b33c7eb5b813cb631b2b0ca5c4029e1788a09bde

Problem:    Comparing functions and partials doesn't work well.
Solution:   Add tests. (Nikolai Pavlov)  Compare the dict and arguments in the
            partial.

https://github.com/vim/vim/commit/8e759ba8651428995b338b66c615367259f79766

Problem:    get() works for Partial but not for Funcref.
Solution:   Accept Funcref.  Also return the function itself. (Nikolai Pavlov)

https://github.com/vim/vim/commit/03e19a04ac2ca55643663b97b6ab94043233dcbd

Problem:    Leaking memory when there is a cycle involving a job and a
            partial.
Solution:   Add a copyID to job and channel.  Set references in items referred
            by them.  Go through all jobs and channels to find unreferenced
            items.  Also, decrement reference counts when garbage collecting.

https://github.com/vim/vim/commit/107e1eef1df3b786ad3ad49fbdb9e058649303b5

Problem:    string() doesn't handle a partial.
Solution:   Make a string from a partial.

https://github.com/vim/vim/commit/5c29154b521e9948190be653cfda666ecbb63b5b

Problem:    Combining dict and args with partial doesn't always work.
Solution:   Use the arguments from the partial.

https://github.com/vim/vim/commit/9e63f61cb01c70fd71652f54b2d01ee27b2a3534