[#319] Handle natural numbers

Description

Handling natural numbers raises Underflow exception. Namely there are two cases:

1. Underflow while performing AND operation of int and nat.

   We can do AND with a negative Integer, so we must apply bitwise-and (.&. from Data.Bits) first and then convert the result to Natural (because the type of "return value" is Natural). However, there is converting Integer to Natural first, so fromIntegral raises Underflow. To solve this, we just fixed the order of the operations.

2. Underflow while unpacking packed int as nat.

   In Michelson we are able to pack a number as int and then to unpack it as nat. When a number is negative, there must be NONE. However, there is an attempt to decode a negative number as Natural in decodeInt function, so Underflow is raised by fromIntegral again. To solve this, we changed fromIntegral to toIntegralSized and made decodeInt return Maybe with Nothing in case of negative number.

Related issue(s)

Resolves #319 (closed)

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