O(n) instead of O(n^2) string concactenation (#54)

Ported my optimisation from FiOne
[#19](Rerumu/FiOne#19)

TLDR; concat function is always faster even with pcall (at least in
Luau) Rerumu/FiOne#19 (comment)

Source.lua

@@ -14,6 +14,7 @@ local table_unpack = table.unpack
 local table_create = table.create
 local table_insert = table.insert
 local table_remove = table.remove
+local table_concat = table.concat
 
 local coroutine_create = coroutine.create
 local coroutine_yield = coroutine.yield
@@ -1018,10 +1019,17 @@ local function luau_load(module, env, luau_settings)
 					local value = stack[inst.B]
 					stack[inst.A] = if value then value else inst.K or false
 				elseif op == 49 then --[[ CONCAT ]]
-					local s = ""
-					for i = inst.B, inst.C do
-						s ..= stack[i]
+					local B, C = inst.B, inst.C
+					local success, s = pcall(table_concat, stack, "", B, C)
+
+					if not success then
+						str = stack[B]
+
+						for i = B + 1, C do
+							str ..= stack[i]
+						end
 					end
+
 					stack[inst.A] = s
 				elseif op == 50 then --[[ NOT ]]
 					stack[inst.A] = not stack[inst.B]