Use lua generator in place of ex_cmds_defs header trick

Closes #788
Fixes #379
Ref #549

scripts/genex_cmds.lua

@@ -0,0 +1,87 @@
+local nvimsrcdir = arg[1]
+local includedir = arg[2]
+local autodir = arg[3]
+
+if nvimsrcdir == '--help' then
+  print ([[
+Usage:
+  lua genex_cmds.lua src/nvim build/include build/src/nvim/auto
+
+Will generate files build/include/ex_cmds_enum.generated.h with cmdidx_T 
+enum and build/src/nvim/auto/ex_cmds_defs.generated.h with main Ex commands 
+definitions.
+]])
+  os.exit(0)
+end
+
+package.path = nvimsrcdir .. '/?.lua;' .. package.path
+
+local enumfname = includedir .. '/ex_cmds_enum.generated.h'
+local defsfname = autodir .. '/ex_cmds_defs.generated.h'
+
+local enumfile = io.open(enumfname, 'w')
+local defsfile = io.open(defsfname, 'w')
+
+local defs = require('ex_cmds')
+local lastchar = nil
+
+local i
+local cmd
+local first = true
+local prevfirstchar = nil
+
+local byte_a = string.byte('a')
+local byte_z = string.byte('z')
+
+local cmdidxs = string.format([[
+static cmdidx_T cmdidxs[%u] = {
+]], byte_z - byte_a + 2)
+
+enumfile:write([[
+typedef enum CMD_index {
+]])
+defsfile:write([[
+static CommandDefinition cmdnames[] = {
+]])
+for i, cmd in ipairs(defs) do
+  local enumname = cmd.enum or ('CMD_' .. cmd.command)
+  firstchar = string.byte(cmd.command)
+  if firstchar ~= prevfirstchar then
+    if (not prevfirstchar
+        or (byte_a <= firstchar      and firstchar     <= byte_z)
+        or (byte_a <= prevfirstchar  and prevfirstchar <= byte_z)) then
+      if not first then
+        cmdidxs = cmdidxs .. ',\n'
+      end
+      cmdidxs = cmdidxs .. '  ' .. enumname
+    end
+    prevfirstchar = firstchar
+  end
+  if first then
+    first = false
+  else
+    defsfile:write(',\n')
+  end
+  enumfile:write('  ' .. enumname .. ',\n')
+  defsfile:write(string.format([[
+  [%s] = {
+    .cmd_name = (char_u *) "%s",
+    .cmd_func = &%s,
+    .cmd_argt = %uL
+  }]], enumname, cmd.command, cmd.func, cmd.flags))
+end
+defsfile:write([[
+
+};
+]])
+enumfile:write([[
+  CMD_SIZE,
+  CMD_USER = -1,
+  CMD_USER_BUF = -2
+} cmdidx_T;
+]])
+cmdidxs = cmdidxs .. [[
+
+};
+]]
+defsfile:write(cmdidxs)