Support Decimal and MutableMapping encoding

For `decimal.Decimal` we can just encode as a `String` and leave it up
to the user how to decode the value on load. For `dict` it's quite handy
to just render any type that quacks like a `MutableMapping` to a table.

Resolves python-poetry#288 and python-poetry#289 but still needs tests!

tomlkit/items.py

@@ -3,10 +3,12 @@
 import re
 import string
 
+from collections.abc import MutableMapping
 from datetime import date
 from datetime import datetime
 from datetime import time
 from datetime import tzinfo
+from decimal import Decimal
 from enum import Enum
 from typing import TYPE_CHECKING
 from typing import Any
@@ -74,6 +76,13 @@ def item(
     ...
 
 
+@overload
+def item(
+    value: Decimal, _parent: Optional["Item"] = ..., _sort_keys: bool = ...
+) -> "String":
+    ...
+
+
 @overload
 def item(
     value: float, _parent: Optional["Item"] = ..., _sort_keys: bool = ...
@@ -167,24 +176,29 @@ def item(
         return Bool(value, Trivia())
     elif isinstance(value, int):
         return Integer(value, Trivia(), str(value))
+    elif isinstance(value, Decimal):
+        return String.from_raw(str(value))
     elif isinstance(value, float):
         return Float(value, Trivia(), str(value))
-    elif isinstance(value, dict):
+    elif isinstance(value, MutableMapping):
         table_constructor = (
             InlineTable if isinstance(_parent, (Array, InlineTable)) else Table
         )
         val = table_constructor(Container(), Trivia(), False)
         for k, v in sorted(
             value.items(),
-            key=lambda i: (isinstance(i[1], dict), i[0] if _sort_keys else 1),
+            key=lambda i: (
+                isinstance(i[1], MutableMapping),
+                i[0] if _sort_keys else 1
+            ),
         ):
             val[k] = item(v, _parent=val, _sort_keys=_sort_keys)
 
         return val
     elif isinstance(value, (list, tuple)):
         if (
             value
-            and all(isinstance(v, dict) for v in value)
+            and all(isinstance(v, MutableMapping) for v in value)
             and (_parent is None or isinstance(_parent, Table))
         ):
             a = AoT([])
@@ -194,12 +208,15 @@ def item(
             table_constructor = InlineTable
 
         for v in value:
-            if isinstance(v, dict):
+            if isinstance(v, MutableMapping):
                 table = table_constructor(Container(), Trivia(), True)
 
                 for k, _v in sorted(
                     v.items(),
-                    key=lambda i: (isinstance(i[1], dict), i[0] if _sort_keys else 1),
+                    key=lambda i: (
+                        isinstance(i[1], MutableMapping),
+                        i[0] if _sort_keys else 1
+                    ),
                 ):
                     i = item(_v, _parent=table, _sort_keys=_sort_keys)
                     if isinstance(table, InlineTable):