Support for Polymorphism

[ghaith]

This is the initial work to support polymorphism in function blocks/classes

The commit introduces a virtual table field on the top level function block in an inheritance chain which points to a globally declared vtable.
Each class will then have its own assigned vtable with method pointers to concrete implementations.
On initialization, the vtable for the concrete type is assigned to the variable.

This does break the function block calling convension but there was no way around it.

Steps to do:

• Create a Vtable type for each class/subclass/interface
  • Create global variables for each new type's vtable
  • Populate the vtable on initialization with the methods from the concrete type
• When initializing an instance, assign the correct vtable
• Adjust method calls to use polymorphism
  • Internal method calls reference the vtable
  • External calls done using a pointer to the function block/class are done using the vtable
  • External calls done not using a pointer are called directly.

Interfaces will be a different PR

[codecov]

Codecov Report

Attention: Patch coverage is 99.35185% with 7 lines in your changes missing coverage. Please review.

Project coverage is 91.29%. Comparing base (e731987) to head (2dd01f8).
Report is 67 commits behind head on master.

Files with missing lines	Patch %	Lines
src/resolver.rs	66.66%	5 Missing ⚠️
src/typesystem.rs	93.33%	1 Missing ⚠️
src/validation/statement.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1474      +/-   ##
==========================================
- Coverage   93.67%   91.29%   -2.38%     
==========================================
  Files         166      174       +8     
  Lines       50097    57498    +7401     
==========================================
+ Hits        46928    52493    +5565     
- Misses       3169     5005    +1836     

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