Remove PResult

[ulmer-a]

This will close #196

This PR will remove all PResults and all unneeded Results from the parser.

The changes that lead to a dead end can be found in PR #202.

[codecov]

Codecov Report

Merging #211 (1fcbc38) into master (5e97491) will increase coverage by 0.00%.
The diff coverage is 97.94%.

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   95.93%   95.94%           
=======================================
  Files          41       41           
  Lines       15430    15449   +19     
=======================================
+ Hits        14803    14822   +19     
  Misses        627      627           

Impacted Files	Coverage Δ
src/codegen/tests.rs	92.85% <ø> (ø)
src/lexer.rs	95.32% <ø> (ø)
src/parser.rs	97.38% <94.95%> (-1.25%)	⬇️
src/index/tests/index_tests.rs	99.57% <100.00%> (ø)
src/lib.rs	52.61% <100.00%> (-0.17%)	⬇️
src/parser/control_parser.rs	95.88% <100.00%> (+3.93%)	⬆️
src/parser/expressions_parser.rs	97.24% <100.00%> (+0.07%)	⬆️

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[ulmer-a]

Ad expression_parser: At this point, parse_expression() will always return a Statement. In case of a syntax error, this will be the whole AST of the expression with the erroneous token replaced with an EmptyStatemen and a diagnostic logged. This is different from it returnnig just a single EmptyStatement, which is not an error condition

[riederm]

nice, we should try to have more code-documentation for the future!
thx!