refactor: small perf win by eliminating string object keys

lua/oil/constants.lua

@@ -1,10 +1,13 @@
 local M = {}
 
-M.FIELD = {
-  id = 1,
-  name = 2,
-  type = 3,
-  meta = 4,
-}
+---Store entries as a list-like table for maximum space efficiency and retrieval speed.
+---We use the constants below to index into the table.
+---@alias oil.InternalEntry any[]
+
+-- Indexes into oil.InternalEntry
+M.FIELD_ID = 1
+M.FIELD_NAME = 2
+M.FIELD_TYPE = 3
+M.FIELD_META = 4
 
 return M