Implement some primitive types methods

compiler/erg_compiler/lib/std/_erg_array.py

@@ -1,4 +1,8 @@
 class Array(list):
+    def dedup(self):
+        return Array(list(set(self)))
+    def dedup_by(self, f):
+        return Array(list(set(map(f, self))))
     def push(self, value):
         self.append(value)
         return self

compiler/erg_compiler/lib/std/_erg_nat.py

@@ -10,3 +10,9 @@ class Nat(int):
     def times(self, f):
         for _ in range(self):
             f()
+
+    def saturating_sub(self, other: Nat):
+        if self > other:
+            return self - other
+        else:
+            return 0

compiler/erg_compiler/lib/std/_erg_str.py

@@ -8,3 +8,32 @@ class Str(str):
             return Str(s)
         else:
             return Error("Str can't be other than str")
+    def get(self, i: int):
+        if len(self) > i:
+            return Str(self[i])
+        else:
+            return None
+
+class StrMut(Str):
+    def try_new(s: str):
+        if isinstance(s, str):
+            return StrMut(s)
+        else:
+            return Error("Str! can't be other than str")
+    def clear(self):
+        self = ""
+    def pop(self):
+        if len(self) > 0:
+            last = self[-1]
+            self = self[:-1]
+            return last
+        else:
+            return Error("Can't pop from empty `Str!`")
+    def push(self, c: str):
+        self += c
+    def remove(self, idx: int):
+        char = self[idx]
+        self = self[:idx] + self[idx+1:]
+        return char
+    def insert(self, idx: int, c: str):
+        self = self[:idx] + c + self[idx:]