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/functional/options/chars_spec.lua

@@ -6,6 +6,7 @@ local eq = helpers.eq
 local exc_exec = helpers.exc_exec
 local insert = helpers.insert
 local feed = helpers.feed
+local meths = helpers.meths
 
 describe("'fillchars'", function()
   local screen
@@ -53,10 +54,18 @@ describe("'fillchars'", function()
       ]])
     end)
 
+    it('supports composing multibyte char', function()
+      command('set fillchars=eob:å̲')
+      screen:expect([[
+        ^                         |
+        å̲                        |*3
+                                 |
+      ]])
+    end)
+
     it('handles invalid values', function()
       shouldfail('eob:') -- empty string
       shouldfail('eob:馬') -- doublewidth char
-      shouldfail('eob:å̲') -- composing chars
       shouldfail('eob:xy') -- two ascii chars
       shouldfail('eob:\255', 'eob:<ff>') -- invalid UTF-8
     end)
@@ -178,4 +187,28 @@ describe("'listchars'", function()
                                                         |
     ]])
   end)
+
+  it('supports composing chars', function()
+    screen:set_default_attr_ids {
+      [1] = { foreground = Screen.colors.Blue1, bold = true },
+    }
+    feed('i<tab><tab><tab>x<esc>')
+    command('set list laststatus=0')
+    -- tricky: the tab value forms three separate one-cell chars,
+    -- thus it should be accepted despite being a mess.
+    command('set listchars=tab:d̞̄̃̒̉̎ò́̌̌̂̐l̞̀̄̆̌̚,eol:å̲')
+    screen:expect([[
+      {1:d̞̄̃̒̉̎ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐l̞̀̄̆̌̚d̞̄̃̒̉̎ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐l̞̀̄̆̌̚d̞̄̃̒̉̎ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐l̞̀̄̆̌̚}^x{1:å̲}                        |
+      {1:~                                                 }|*3
+                                                        |
+    ]])
+
+    meths._invalidate_glyph_cache()
+    screen:_reset()
+    screen:expect([[
+      {1:d̞̄̃̒̉̎ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐l̞̀̄̆̌̚d̞̄̃̒̉̎ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐l̞̀̄̆̌̚d̞̄̃̒̉̎ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐ò́̌̌̂̐l̞̀̄̆̌̚}^x{1:å̲}                        |
+      {1:~                                                 }|*3
+                                                        |
+    ]])
+  end)
 end)