Only call exec once per group

Calls to `exec` are quite expensive, so using
string concatenation and a single exec at the end
instead of a couple of exec yields a nice
startup performance boost.

colors/nord.vim

@@ -136,23 +136,27 @@ if !exists("g:nord_bold_vertical_split_line")
 endif
 
 function! s:hi(group, guifg, guibg, ctermfg, ctermbg, attr, guisp)
+  let cmd = ""
   if a:guifg != ""
-    exec "hi " . a:group . " guifg=" . a:guifg
+    let cmd = cmd . " guifg=" . a:guifg
   endif
   if a:guibg != ""
-    exec "hi " . a:group . " guibg=" . a:guibg
+    let cmd = cmd . " guibg=" . a:guibg
   endif
   if a:ctermfg != ""
-    exec "hi " . a:group . " ctermfg=" . a:ctermfg
+    let cmd = cmd . " ctermfg=" . a:ctermfg
   endif
   if a:ctermbg != ""
-    exec "hi " . a:group . " ctermbg=" . a:ctermbg
+    let cmd = cmd . " ctermbg=" . a:ctermbg
   endif
   if a:attr != ""
-    exec "hi " . a:group . " gui=" . a:attr . " cterm=" . substitute(a:attr, "undercurl", s:underline, "")
+    let cmd = cmd . " gui=" . a:attr . " cterm=" . substitute(a:attr, "undercurl", s:underline, "")
   endif
   if a:guisp != ""
-    exec "hi " . a:group . " guisp=" . a:guisp
+    let cmd = cmd . " guisp=" . a:guisp
+  endif
+  if cmd != ""
+    exec "hi " . a:group . cmd
   endif
 endfunction
 
@@ -225,12 +229,12 @@ if has('nvim')
   call s:hi("DiagnosticUnderlineError" , s:nord11_gui, "", s:nord11_term, "", "undercurl", "")
   call s:hi("DiagnosticUnderlineInfo" , s:nord8_gui, "", s:nord8_term, "", "undercurl", "")
   call s:hi("DiagnosticUnderlineHint" , s:nord10_gui, "", s:nord10_term, "", "undercurl", "")
-  
+
   "+- Neovim DocumentHighlight -+
   call s:hi("LspReferenceText", "", s:nord3_gui, "", s:nord3_term, "", "")
   call s:hi("LspReferenceRead", "", s:nord3_gui, "", s:nord3_term, "", "")
   call s:hi("LspReferenceWrite", "", s:nord3_gui, "", s:nord3_term, "", "")
-  
+
   "+- Neovim LspSignatureHelp -+
   call s:hi("LspSignatureActiveParameter", s:nord8_gui, "", s:nord8_term, "", s:underline, "")
 endif
@@ -669,7 +673,7 @@ if exists('g:loaded_clap')
   call s:hi("ClapNoMatchesFound", s:nord13_gui, "", s:nord13_term, "", "", "")
   call s:hi("ClapSelected", s:nord7_gui, "", s:nord7_term, "", s:bold, "")
   call s:hi("ClapSelectedSign", s:nord9_gui, "", s:nord9_term, "", "", "")
-  
+
   let s:clap_matches = [
           \ [s:nord8_gui,  s:nord8_term] ,
           \ [s:nord9_gui,  s:nord9_term] ,
@@ -681,7 +685,7 @@ if exists('g:loaded_clap')
     call s:hi("ClapFuzzyMatches" . s:nord_clap_match_i, clap_match_color[0], "", clap_match_color[1], "", "", "")
   endfor
   unlet s:nord_clap_match_i
-  
+
   hi! link ClapCurrentSelection PmenuSel
   hi! link ClapCurrentSelectionSign ClapSelectedSign
   hi! link ClapInput Pmenu
@@ -769,7 +773,7 @@ if exists('g:vim_pandoc_syntax_exists')
   hi! link pandocTableHeaderWord pandocAtxHeader
   hi! link pandocUListItemBullet Operator
 endif
-  
+
 if has('nvim')
   " tree-sitter
   " > nvim-treesitter/nvim-treesitter
@@ -882,13 +886,13 @@ if exists('g:loaded_vimwiki')
       call s:hi("VimwikiHeader".s:i, s:vimwiki_hcolor_guifg[s:i-1] , "", s:vimwiki_hcolor_ctermfg[s:i-1], "", s:bold, "")
     endfor
   endif
-  
+
   call s:hi("VimwikiLink", s:nord8_gui, "", s:nord8_term, "", s:underline, "")
   hi! link VimwikiHeaderChar markdownHeadingDelimiter
   hi! link VimwikiHR Keyword
   hi! link VimwikiList markdownListMarker
 endif
-  
+
 " YAML
 " > stephpy/vim-yaml
 call s:hi("yamlKey", s:nord7_gui, "", s:nord7_term, "", "", "")