Add luacheck + fix warnings detected by it

.luacheckrc

@@ -0,0 +1,4 @@
+std = "lua51+luajit+minetest"
+
+globals = {"test_string_to_table"}
+read_globals = {"DIR_DELIM"}

alg1.lua

@@ -1,9 +1,8 @@
 local function stringToTable(str)
     local parseTable -- defined later, but we need it in parseValue
 
-    local function parseValue(s, i)
-        s = s:match("^%s*(.-)%s*$", i) -- Trim leading/trailing spaces
-        i = 1
+    local function parseValue(s, idx)
+        s = s:match("^%s*(.-)%s*$", idx) -- Trim leading/trailing spaces
 
         if s:sub(1, 1) == "{" then
             return parseTable(s)
@@ -47,7 +46,6 @@ local function stringToTable(str)
 
     local function parseKey(s, i)
         s = s:match("^%s*(.-)%s*$", i) -- Trim leading/trailing spaces
-        i = 1
         if s:find("^[%a_]") then
             local key = s:match("^[%a%d_]*")
             if key ~= "true" and key ~= "false" and key ~= "nil" then

alg2.lua

@@ -55,7 +55,6 @@ local function codedStringToTable(str)
     local tbl = {}
     local key, value
     local next_key = 1
-    local inKey = true
     local depth = 0
     local buffer = ""
 

algtour.lua

@@ -95,7 +95,7 @@ local function stringToTable(str)
     local success = true
     local tbl
 
-    local replacements = 0
+    local replacements
     local table_count = 0
     repeat
         str, replacements = str:gsub("({[^{}]*})", function (match)