Add Lua code for chapter 6-9 (#36)

* Add Lua code for chapter 6

* Add Lua code for chapter 7

* Add Lua code for chapter 8

* Add Lua code for chapter 9

06_breadth-first_search/lua/01_breadth-first_search.lua

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+-- Custom deque module
+require "deque"
+
+local function person_is_seller(name)
+    return string.char(string.byte(name, -1)) == "m"
+end
+
+local graph = {}
+graph["you"] = {"alice", "bob", "claire"}
+graph["bob"] = {"anuj", "peggy"}
+graph["alice"] = {"peggy"}
+graph["claire"] = {"thom", "jonny"}
+graph["anuj"] = {}
+graph["peggy"] = {}
+graph["thom"] = {}
+graph["jonny"] = {}
+
+local function search(name)
+    local search_queue = deque:new()
+    for _, value in pairs(graph[name]) do
+        search_queue:push_right(value)
+    end
+    -- This array is how you keep track of which people you've searched before.
+    local searched = {}
+    while search_queue:len() > 0 do
+        local person = search_queue:pop_left()
+        -- Only search this person if you haven't already searched them.
+        if not searched[person] then
+            if person_is_seller(person) then
+                print(person .. " is a mango seller!")
+                return true
+            else
+                for _, value in pairs(graph[person]) do
+                    search_queue:push_right(value)
+                end
+                -- Marks this person as searched
+                searched[person] = true
+            end
+        end
+    end
+    return false
+end
+
+search("you")

06_breadth-first_search/lua/deque.lua

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+deque = {}
+
+function deque:new()
+    self.__index = self
+    return setmetatable({first = 0, last = -1}, self)
+end
+
+function deque:len()
+    return self.last - self.first + 1
+end
+
+function deque:push_left(value)
+    local first = self.first - 1
+    self.first = first
+    self[first] = value
+end
+
+function deque:push_right(value)
+    local last = self.last + 1
+    self.last = last
+    self[last] = value
+end
+
+function deque:pop_left()
+    local first = self.first
+    if first > self.last then
+        error "deque is empty"
+    end
+    local value = self[first]
+    self[first] = nil
+    self.first = first + 1
+    return value
+end
+
+function deque:pop_right()
+    local last = self.last
+    if self.first > last then
+        error "deque is empty"
+    end
+    local value = self[last]
+    self[last] = nil
+    self.last = last - 1
+    return value
+end
+
+return deque