Support for Abstracts

[VincentRPS]

Add support for Abstracts, shared function signatures with different functionality
based on struct or enum type, similar to Rust's traits.

NOTE: Generics

While Generics aren't explicitly needed for implementing Abstracts, they are very
useful for users in making them and in many cases traits can't be made without
generics.

NOTE: Special Support

The compiler due to backend limitations is expected to provide special support for the
following:

• Add (Blocked by Generics) (Usage of +)
• Sub (Blocked by Generics) (Usage of -)
• Div (Blocked by Generics) (Usage of /)
• Mul (Blocked by Generics) (Usage of *)
• other mathematical symbols (probably also blocked by Generics)
• Eq (Blocked by Generics) (Usage of ==)

[elenakrittik]

Just to clarify: the functionality ("purpose", in other words) of abstract functions is the same; it's the implementation that differs