Introduce ProgramPartParser to simplify construction of parts of programs

[rtaupe]

Some unit tests already have been adapted to use the new parser for Literals instead of using various constructors, which makes the unit tests easier to understand.

[codecov]

Codecov Report

Merging #248 into master will increase coverage by 0.07%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #248      +/-   ##
============================================
+ Coverage     76.83%   76.91%   +0.07%     
- Complexity     2056     2063       +7     
============================================
  Files           150      151       +1     
  Lines          6782     6814      +32     
  Branches       1184     1189       +5     
============================================
+ Hits           5211     5241      +30     
  Misses         1177     1177              
- Partials        394      396       +2     

Impacted Files	Coverage Δ	Complexity Δ
...en/kr/alpha/grounder/parser/ProgramPartParser.java	83.33% <83.33%> (ø)	6.00 <6.00> (?)
...a/at/ac/tuwien/kr/alpha/grounder/Substitution.java	77.33% <100.00%> (+1.38%)	29.00 <1.00> (-1.00)	⬆️
...in/java/at/ac/tuwien/kr/alpha/common/Interner.java	90.00% <0.00%> (-10.00%)	3.00% <0.00%> (-1.00%)
.../at/ac/tuwien/kr/alpha/grounder/NaiveGrounder.java	88.34% <0.00%> (+0.52%)	109.00% <0.00%> (+3.00%)

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