Problem:    Cannot get composing characters from the screen.
Solution:   Add screenchars() and screenstring(). (partly by Ozaki Kiichi,
            closes vim/vim#4059)
https://github.com/vim/vim/commit/2912abb3a2fd72074e3901c8ae1d4a77ce764675

Problem:    Recognizing octal numbers is confusing.
Solution:   Introduce scriptversion 4: do not use octal and allow for single
            quote inside numbers.
https://github.com/vim/vim/commit/60a8de28d11595f4df0419ece8afa7d6accc9fbd

:scriptversion is N/A.

Cherry-pick Test_readfile_binary() from v8.1.0742.

Note that this patch was missing vim_str2nr() changes, and so fails the
tests; this was fixed in v8.1.2036.

Cannot be fully ported as chdir() hasn't been ported yet.

Problem:    More functions can be used as methods.
Solution:   Make a few more functions usable as a method.
https://github.com/vim/vim/commit/64b4d73524b9a2304d89b87529cd8d3cef14b856

Note that the old-style version of Test_byteidx() was already translated
to a Lua test in 069_multibyte_formatting_spec.lua. Keep both versions,
using Test_byteidx() to mainly test the method call syntax for byteidx()
and byteidxcomp().

Problem:    More functions can be used as methods.
Solution:   Make a few more functions usable as a method.
https://github.com/vim/vim/commit/e49fbff384e45dd17fed72321c26937edf6de16b

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
https://github.com/vim/vim/commit/073e4b92e613d22ce7b16e0fbf5c0e40cb5f9b2c

test_popup.vim already has the changes from this patch (they're N/A
anyway).

Problem:    More functions can be used as methods.
Solution:   Make float functions usable as a method.
https://github.com/vim/vim/commit/93cf85f9ef02931de3f8c8e536a137da0b48b7dc

Fix atan2() doc typo (patch referred to it as atan()).
Adjust Test_fmod() method test to expect "str2float('nan')".

Problem:    Only some assert functions can be used as a method.
Solution:   Allow using most assert functions as a method.
https://github.com/vim/vim/commit/24278d2407dfbc8d93eb36593cdd006ff5d86f94

Port tests to assert_spec.lua.

Problem:    Cannot use printf() as a method.
Solution:   Pass the base as the second argument to printf().
https://github.com/vim/vim/commit/fd8ca21b3ff207e44891aef922935d4adcd140cf