Support case insesitive keywords

[ghaith]

Add support for keywords and identifiers written using different cases
Closes #246

TODO:

• Keywords
• Identifiers

[ghaith]

I generally don't like the way this is done atm because it is not really configurable and might be hard to extend/maintain on the long run. I don't see how else we can do it otherwise though. So I'm open to suggestions
I still like the Identifier idea in the index, which is not yet in. But the lexing seems strange here.

[codecov]

Codecov Report

Merging #255 (65996a9) into master (7103769) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #255   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files          39       39           
  Lines       10546    10555    +9     
=======================================
+ Hits        10039    10048    +9     
  Misses        507      507           

Impacted Files	Coverage Δ
src/lexer.rs	95.55% <ø> (ø)
src/codegen/generators/data_type_generator.rs	93.06% <100.00%> (ø)
src/codegen/llvm_index.rs	94.96% <100.00%> (+0.12%)	⬆️
src/index.rs	94.59% <100.00%> (+0.04%)	⬆️
src/index/visitor.rs	95.94% <100.00%> (+0.03%)	⬆️

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[riederm]

looks good ... yes we gonna need to keep adding new keywords to these tests :-/
but I dont see any other approach with the lexer we use.

I added two additional issue to track further work

• make case insensitive parser configurable #256
• performance-analysis: case-insensitive parser #257