Problem:    The eval.c file is too big.
Solution:   Move code related to variables to evalvars.c. (Yegappan
            Lakshmanan, closes vim/vim#4868)
https://github.com/vim/vim/commit/0522ba0359c96a8c2a4fc8fca0d3b58e49dda759

Name the new file eval/vars.c instead.

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459

Problem:    Giving error messages is not flexible.
Solution:   Add semsg().  Change argument from "char_u *" to "char *", also
            for msg() and get rid of most MSG macros. (Ozaki Kiichi, closes
            vim/vim#3302)  Also make emsg() accept a "char *" argument.  Get rid of
            an enormous number of type casts.
https://github.com/vim/vim/commit/f9e3e09fdc93be9f0d47afbc6c7df1188c2a5a0d

Problem:    Cannot handle binary data.
Solution:   Add the Blob type. (Yasuhiro Matsumoto, closes vim/vim#3638)
https://github.com/vim/vim/commit/6e5ea8d2a995b32bbc5972edc4f827b959f2702f

Nvim-specific Blob conversions are implemented in future commits.

Refactor write_blob() to use a FileDescriptor, as f_writefile() was
refactored to use one (does not apply to read_blob()).

Use var_check_lock() in f_add() for Blobs from v8.1.0897.

Add a modeline to test_blob.vim and fix some doc typos.

Include if_perl.txt's VIM::Blob() documentation. Interestingly, this
function already worked before this port, as it just returns a Blob
string literal, not an actual Blob object.

N/A patches for version.c:

vim-patch:8.1.0741: viminfo with Blob is not tested

Problem:    Viminfo with Blob is not tested.
Solution:   Extend the viminfo test.  Fix reading a blob.  Fixed storing a
            special variable value.
https://github.com/vim/vim/commit/8c8b8bb56c724cc1bfc3d8520eec33f2d399697c

vim-patch:8.1.1022: may use NULL pointer when out of memory

Problem:    May use NULL pointer when out of memory. (Coverity)
Solution:   Check for blob_alloc() returning NULL.
https://github.com/vim/vim/commit/e142a9467a7f6845a426d8db6efedf246d3c13ac

assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.

This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.

Problem:    VAR_SPECIAL is also used for booleans.
Solution:   Add VAR_BOOL for better type checking.
https://github.com/vim/vim/commit/9b4a15d5dba354d2e1e02871470bad103f34769a

Problem:    Floating point exception with "%= 0" and "/= 0".
Solution:   Avoid dividing by zero. (Dominique Pelle, closes vim/vim#4058)
https://github.com/vim/vim/commit/e21c1580b7acb598a6e3c38565434fe5d0e2ad7a