Problem:    spell.c is too big.
Solution:   Split it in spell file handling and spell checking.

Problem:    eval.c is still too big.
Solution:   Split off internal functions to evalfunc.c.

Problem:    Non-Unix builds fail.
Solution:   Update Makefiles for new files.

Problem:    farsi.c and arabic.c are included in a strange way.
Solution:   Build them like other files.

Problem:    No support for JSON.
Solution:   Add jsonencode() and jsondecode().  Also add v:false, v:true,
            v:null and v:none.

Problem:    Can't build on MS-Windows.
Solution:   Include the new files in all the Makefiles.