Improve implementation of literal values

[ShivamSarodia]

Today, ILValues have a literal_val attribute that stores None if the ILValue does not have a known compile-time integer literal value, or the integer value as a Python number if it does.

A better implementation of literal values would involve a LiteralValue class, from which IntegerLiteralValue, StringLiteralValue, etc. would inherit. Then, the literal_val attribute of an IL value that has a fixed value known at compile time would be an instance of a subclass of LiteralValue.

[ShivamSarodia]

@christian-stephen is taking a look at this issue :)

[ShivamSarodia]

#98 did most of this, but there are still some potential improvements to explore. Specifically, it may be interesting to explore adding a raw() function to the _Literal object. Then, we write something like literal.raw() to get a raw representation of the literal value that can be directly inserted into ASM code.

Also, the code in #98 assumes that the type of the literal for an IL value will be compatible with the type of the IL value itself; that is, an IL value of integral type will have a literal_val of only integral type. While this works for now, I'm not sure it's a fair assumption going forwards. To protect for this, we can add a test for int array[5]; array[(int)"hello"]