allow assignments between different pointer types

[riederm]

casts between pointer types will generate a llvm bitcast statement

e.g.
x : INT; p : POINTER TO BYTE;
p := &x;

fixes #463

[riederm]

with this commit we can build the oscat lib as it is commited to https://github.com/PLC-lang/oscat.git (#f72b5bf)

cargo run -- -c -o oscat.o path/to/oscat-repo/*.st

althoug it does not mean that the compiled code is correct - we can now compile without segfaulting. clang-12 still has some issues (but very late in the code ~line 46k)

[codecov-commenter]

Codecov Report

Merging #466 (e1de25f) into master (4a7b8a6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
- Coverage   92.78%   92.78%   -0.01%     
==========================================
  Files          36       36              
  Lines       13216    13220       +4     
==========================================
+ Hits        12263    12266       +3     
- Misses        953      954       +1     

Impacted Files	Coverage Δ
src/codegen/llvm_typesystem.rs	79.91% <100.00%> (+0.33%)	⬆️
src/lexer.rs	97.66% <0.00%> (-0.39%)	⬇️

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