Add SingleValueExpression to type ValueExpressions that always return a single value

[jvdb]

This PR merges to the branch of #282. So please review that first.

Introduces SingleValueExpression as proposed in #70.

Instead of the ImmutableList<Value> that is returned by ValueExpression.eval(...), SingleValueExpression has a evalSingle(...) method with the same arguments that returns an Optional<Value>. SingleValueExpression extends ValueExpression, so that it can be used wherever a ValueExpression is required.

The default implementation of the eval(...) method on SingleValueExpression converts an empty result of evalSingle(...) to an empty list, otherwise the returned value is stored as single value in the returned list.

Converted to SingleValueExpression are:

1. Token arguments:
   1. the size argument of Def
   2. the size argument of Nod (essentially just a proxy for Def)
   3. the n argument of RepN
2. ValueExpression implementations:
   1. CurrentOffset
   2. CurrentIteration
   3. Const
   4. First
   5. Last
   6. Count
   7. Self
3. ValueExpression arguments:
   1. the count argument of Expand
   2. the initial argument of Fold

Tasks:

• The Fold implementations themselves should also be converted, not just (some of) the argument(s).
• Resolve static analysis tools results.

The unaddressed tasks that was in the list concerning refactoring the type of Fold's reducer argument now has its own issue: #284.

This PR resolves #70.

[codecov]

Codecov Report

Merging #283 into value-interface will not change coverage.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             value-interface   #283   +/-   ##
================================================
  Coverage                100%   100%           
- Complexity              1043   1046    +3     
================================================
  Files                     93     94    +1     
  Lines                   1406   1399    -7     
  Branches                 154    150    -4     
================================================
- Hits                    1406   1399    -7

Impacted Files	Coverage Δ	Complexity Δ
.../parsingdata/metal/expression/value/CoreValue.java	100% <ø> (ø)	18 <0> (ø)	⬇️
...o/parsingdata/metal/expression/value/FoldLeft.java	100% <ø> (ø)	3 <0> (ø)	⬇️
.../parsingdata/metal/expression/value/FoldRight.java	100% <ø> (ø)	3 <0> (ø)	⬇️
...singdata/metal/expression/value/reference/Nth.java	100% <ø> (ø)	19 <0> (ø)	⬇️
...l/expression/value/reference/CurrentIteration.java	100% <ø> (ø)	15 <0> (-1)	⬇️
...rc/main/java/io/parsingdata/metal/format/JPEG.java	100% <100%> (ø)	1 <0> (ø)	⬇️
...ingdata/metal/expression/value/reference/Last.java	100% <100%> (ø)	8 <0> (ø)	⬇️
...ingdata/metal/expression/value/reference/Self.java	100% <100%> (ø)	5 <2> (ø)	⬇️
...src/main/java/io/parsingdata/metal/token/RepN.java	100% <100%> (ø)	11 <5> (ø)	⬇️
.../src/main/java/io/parsingdata/metal/Shorthand.java	100% <100%> (ø)	131 <33> (ø)	⬇️
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 87e0260...60fedd2. Read the comment docs.

[rdvdijk]

Looks good to me.

The only thing that looks like something that could be improved is that we now need to add last() around a few expressions.

Another thing is that last() will not throw an exception, where the old ValueExpressions would explicitly check for lists of length 1, and throw an exception in the other cases.