[decimal] Updates to the Decimal type (#38)

This PR brings some changes to the `Decimal` type:
- Add augmented arithmetic operations, e.g., `__iadd__`, `__isub__`,
etc.
- Add `repr_components` method.
- Change the trait `Intable` to `IntableRaising`.

Author: forfudan

src/decimojo/constants.mojo

@@ -481,15 +481,15 @@ fn E0D25() -> Decimal:
 # The repr of the magic numbers can be obtained by the following code:
 #
 # ```mojo
-# fn print_repr_from_words(value: String, ln_value: String) raises:
+# fn print_repr_words(value: String, ln_value: String) raises:
 #     """
 #     Prints the hex representation of a logarithm value.
 #     Args:
 #         value: The original value (for display purposes).
 #         ln_value: The natural logarithm as a String.
 #     """
 #     var log_decimal = Decimal(ln_value)
-#     print("ln(" + value + "): " + log_decimal.repr_from_words())
+#     print("ln(" + value + "): " + log_decimal.repr_words())
 # ```