Proj 11  - String: Replace Function

Add replace function
Proj 11-String : Replace,erase,[ ], allocate

void replace(const int start, const int len, const MyString& rhs, const int rhs_start = 0); //Tested,
48	+    void swap(const int pos1, const int pos2); //done, not tested
49	+    bool compare(const MyString& tocpr) const; //done, Tested in find char* function
50	+    bool compare(const char* tocpr) const; //done
51	+    bool compare(const int l_start ,const MyString& tocpr, const int r_start, const int r_len) const; //done, not tested
52	+    bool compare(const int l_start ,const char* tocpr, const int r_start, const int r_len) const; //done
53	+    void insert(const char* to_insrt, const int at_pos);
54	+    void insert(const MyString& to_insrt, const int at_pos);
55	+    void erase(const int start, const int length);  // done, tested, erase some part and reduce its length
56	+    int find(const char x_char, int start = 0) const; //done, Tested
57	+    int find(const char* x_str, int start = 0) const;
58	+    int find_first_of(const char* charset, int start = 0) const; // this is find any, ex: find "aeiou" in "apel"
59	+    int find_last_of(const char* x_char, int start = 0) const;