feat: create singleton events/commands/keymaps for load handlers

Implementing the idea @lewis6991 proposed for handling multiple loads on
the same command/keymap/event added by separate calls to `add`. Changes
the loaders to keep a table of plugins associated with a given load
trigger and only create new events/keymaps/commands for new triggers,
otherwise simply appending to the set of plugins to be loaded on the
given trigger.

One note: we may need to make loaders delete plugins from these lists
after load, to handle cases like:
- add(event, plugins_1)
- event happens, plugins_1 loaded
- add(event, plugins_2)
- event happens, without deleting plugins_1 gets loaded again

I don't believe that this is a potential bug introduced by this
change---it could've happened with the more naive handlers as well

teal/packer/handlers/cmd.tl

@@ -1,26 +1,26 @@
-local util = require 'packer.handlers.util'
+local command_plugins : {string: {Plugin}} = {}
 
 return function(plugins: {string:Plugin}, loader: function({Plugin}))
-  local commands: {string:{Plugin}} = {}
+  local new_commands: {string} = {}
   for _, plugin in pairs(plugins) do
     if plugin.cmd then
       for _, cmd in ipairs(plugin.cmd) do
-        commands[cmd] = commands[cmd] or {}
-        table.insert(commands[cmd], plugin)
+        if not command_plugins[cmd] then
+          command_plugins[cmd] = {}
+          new_commands[#new_commands + 1] = cmd
+        end
+
+        table.insert(command_plugins[cmd], plugin)
       end
     end
   end
 
-  for cmd, cplugins in pairs(commands) do
-
-    util.register_destructor(cplugins, function()
-      vim.api.nvim_del_user_command(cmd)
-    end)
+  for _, cmd in ipairs(new_commands) do
 
     vim.api.nvim_create_user_command(cmd,
       function(args: vim.api.UserCmdParams)
-        loader(cplugins)
-
+        vim.api.nvim_del_user_command(cmd)
+        loader(command_plugins[cmd])
         local lines = args.line1 == args.line2 and '' or (args.line1 .. ',' .. args.line2)
         vim.cmd(string.format(
           '%s %s%s%s %s',

teal/packer/handlers/event.tl

@@ -1,33 +1,30 @@
-local util = require 'packer.handlers.util'
+local event_plugins: {string:{Plugin}} = {}
 
 return function(plugins: {string:Plugin}, loader: function({Plugin}))
-  local events: {string:{Plugin}} = {}
-
+  local new_events: {string} = {}
   for _, plugin in pairs(plugins) do
     if plugin.event then
       for _, event in ipairs(plugin.event) do
-        events[event] = events[event] or {}
-        table.insert(events[event], plugin)
+        if not event_plugins[event] then
+          event_plugins[event] = {}
+          new_events[#new_events + 1] = event
+        end
+
+        table.insert(event_plugins[event], plugin)
       end
     end
   end
 
-  for event, eplugins in pairs(events) do
+  for _, event in ipairs(new_events) do
     -- Handle 'User Foo' events
     local ev, pattern = unpack(vim.split(event, '%s+'))
-
-    local id = vim.api.nvim_create_autocmd(ev, {
+    vim.api.nvim_create_autocmd(ev, {
       pattern = pattern,
       once = true,
       callback = function()
-        loader(eplugins)
+        loader(event_plugins[event])
         vim.api.nvim_exec_autocmds(event, { modeline = false })
       end
     })
-
-    util.register_destructor(eplugins, function()
-      pcall(vim.api.nvim_del_autocmd, id)
-    end)
-
   end
 end