perf: speed up foldexpr by removing vimscript (#37)

Going through the aerial#foldexpr() script was taking some overhead. By
directly using v:lua we can avoid that and get a ~30% speed boost.

autoload/aerial.vim

@@ -1,7 +1,3 @@
-function! aerial#foldexpr() abort
-	return luaeval('require"aerial.fold".foldexpr(_A)', v:lnum)
-endfunction
-
 function! aerial#fzf() abort
 	let l:labels = luaeval("require('aerial.fzf').get_labels()")
 	if type(l:labels) == type(v:null) && l:labels == v:null

lua/aerial/fold.lua

@@ -59,7 +59,7 @@ local function compute_folds(bufnr)
   return fold_levels
 end
 
-M.foldexpr = function(lnum)
+M.foldexpr = function()
   if util.is_aerial_buffer() then
     return "0"
   end
@@ -73,6 +73,7 @@ M.foldexpr = function(lnum)
   if not cache then
     cache = compute_folds(bufnr)
   end
+  local lnum = vim.v.lnum
   -- Clear the cache after calling foldexpr on all lines
   if lnum == vim.api.nvim_buf_line_count(bufnr) then
     fold_cache[bufnr] = nil
@@ -119,7 +120,7 @@ M.maybe_set_foldmethod = function(bufnr)
       vim.api.nvim_win_set_var(winid, prev_fdm, fdm)
       vim.api.nvim_win_set_var(winid, prev_fde, fde)
       vim.api.nvim_win_set_option(winid, "foldmethod", "expr")
-      vim.api.nvim_win_set_option(winid, "foldexpr", "aerial#foldexpr()")
+      vim.api.nvim_win_set_option(winid, "foldexpr", "v:lua.aerial_foldexpr()")
       if config.link_tree_to_folds then
         vim.api.nvim_win_set_option(winid, "foldlevel", 99)
       end

lua/aerial/init.lua

@@ -248,4 +248,6 @@ M.info = function()
   print(string.format("Show symbols: %s", config.get_filter_kind_map()))
 end
 
+_G.aerial_foldexpr = fold.foldexpr
+
 return M

lua/aerial/util.lua

@@ -190,7 +190,7 @@ M.is_floating_win = function(winid)
 end
 
 M.is_managing_folds = function(winid)
-  return vim.api.nvim_win_get_option(winid or 0, "foldexpr") == "aerial#foldexpr()"
+  return vim.api.nvim_win_get_option(winid or 0, "foldexpr") == "v:lua.aerial_foldexpr()"
 end
 
 M.detect_split_direction = function(bufnr)