refactor(options): use schar_T representation for fillchars and listchars

A bit big, but practically it was a lot simpler to change over all
fillchars and all listchars at once, to not need to maintain two
parallel implementations.

This is mostly an internal refactor, but it also removes an arbitrary
limitation: that 'fillchars' and 'listchars' values can only be
single-codepoint characters. Now any character which fits into a single
screen cell can be used.

test/unit/statusline_spec.lua

@@ -9,6 +9,7 @@ local NULL = helpers.NULL
 local buffer = helpers.cimport('./src/nvim/buffer.h')
 local globals = helpers.cimport('./src/nvim/globals.h')
 local stl = helpers.cimport('./src/nvim/statusline.h')
+local grid = helpers.cimport('./src/nvim/grid.h')
 
 describe('build_stl_str_hl', function()
   local buffer_byte_size = 100
@@ -25,8 +26,11 @@ describe('build_stl_str_hl', function()
     output_buffer = to_cstr(string.rep(' ', buffer_byte_size))
 
     local pat = arg.pat or ''
-    local fillchar = arg.fillchar or (' '):byte()
+    local fillchar = arg.fillchar or ' '
     local maximum_cell_count = arg.maximum_cell_count or buffer_byte_size
+    if type(fillchar) == type('') then
+      fillchar = grid.schar_from_str(fillchar)
+    end
 
     return stl.build_stl_str_hl(
       globals.curwin,
@@ -61,7 +65,7 @@ describe('build_stl_str_hl', function()
     -- so we either fill in option with arg or an empty dictionary
     local option = arg or {}
 
-    local fillchar = option.fillchar or (' '):byte()
+    local fillchar = option.fillchar or ' '
     local expected_cell_count = option.expected_cell_count or statusline_cell_count
     local expected_byte_length = option.expected_byte_length or #expected_stl
 
@@ -110,35 +114,35 @@ describe('build_stl_str_hl', function()
     10,
     'abcde%=',
     'abcde!!!!!',
-    { fillchar = ('!'):byte() }
+    { fillchar = '!' }
   )
   statusline_test(
     'should handle `~` as a fillchar',
     10,
     '%=abcde',
     '~~~~~abcde',
-    { fillchar = ('~'):byte() }
+    { fillchar = '~' }
   )
   statusline_test(
     'should put fillchar `!` in between text',
     10,
     'abc%=def',
     'abc!!!!def',
-    { fillchar = ('!'):byte() }
+    { fillchar = '!' }
   )
   statusline_test(
     'should put fillchar `~` in between text',
     10,
     'abc%=def',
     'abc~~~~def',
-    { fillchar = ('~'):byte() }
+    { fillchar = '~' }
   )
   statusline_test(
     'should put fillchar `━` in between text',
     10,
     'abc%=def',
     'abc━━━━def',
-    { fillchar = 0x2501 }
+    { fillchar = '━' }
   )
   statusline_test(
     'should handle zero-fillchar as a space',
@@ -249,7 +253,7 @@ describe('build_stl_str_hl', function()
       expected_stl:gsub('%~', ' '),
       arg
     )
-    arg.fillchar = ('!'):byte()
+    arg.fillchar = '!'
     statusline_test(
       description .. ' with fillchar `!`',
       statusline_cell_count,
@@ -257,7 +261,7 @@ describe('build_stl_str_hl', function()
       expected_stl:gsub('%~', '!'),
       arg
     )
-    arg.fillchar = 0x2501
+    arg.fillchar = '━'
     statusline_test(
       description .. ' with fillchar `━`',
       statusline_cell_count,
@@ -456,7 +460,7 @@ describe('build_stl_str_hl', function()
     10,
     'Ą%=mid%=end',
     'Ą@mid@@end',
-    { fillchar = ('@'):byte() }
+    { fillchar = '@' }
   )
 
   -- escaping % testing