WIP: Attempt on Generic Numerical EqualConstraint

[stevenaw]

Moved the work from @manfred-brands into a shared branch in the nunit repo for collaboration
Contributes to #53

[stevenaw]

There were two failures at the time I had to stop working on this yesterday, shown below.

The second error, where two ValueTuples which should be equal but aren't reporting as such, was failing when trying to use the default Equals() function here:

nunit/src/NUnitFramework/framework/Constraints/EqualConstraint{T}.cs

Line 296 in 149d5b4

isSuccess = a.Equals(_expected);

The first error, it appears an AssertionException should be thrown but isn't as it's going into the fallback comparer in that case.

Errors, Failures and Warnings

1) Failed : NUnit.Framework.Tests.Constraints.EqualConstraintTests.SameValueDifferentTypeExactMessageMatch
  Assert.That(ex?.Message, Does.Contain("  Expected: 0 (Int32)" + NL + "  But was:  0 (IntPtr)" + NL))
  Expected: String containing "  Expected: 0 (Int32)
  But was:  0 (IntPtr)
"
  But was:  "  Assert.That(new IntPtr(0), Is.EqualTo(0))
  Expected: 0
  But was:  0
"
   at NUnit.Framework.Tests.Constraints.EqualConstraintTests.SameValueDifferentTypeExactMessageMatch() in C:\Users\sweer\Documents\Development\OSS-stevenaw\nunit\src\NUnitFramework\tests\Constraints\EqualConstraintTests.cs:line 838

2) Failed : NUnit.Framework.Tests.Constraints.ValueTupleEqualityTests.ValueTupleElementsAreComparedUsingNUnitEqualityComparer
  Assert.That(a, Is.EqualTo(b))
  Expected: (<empty>)
  But was:  (<empty>)
   at NUnit.Framework.Tests.Constraints.ValueTupleEqualityTests.ValueTupleElementsAreComparedUsingNUnitEqualityComparer() in C:\Users\sweer\Documents\Development\OSS-stevenaw\nunit\src\NUnitFramework\tests\Constraints\ValueTupleEqualityTests.cs:line 74

[stevenaw]

Update: I tried my hand at an IResolvableConstraint<T> and IConstraint<T> in order to support a signature like Assert.That(TActual, IResolvableConstraint<TActual>) so that we could get compile-time wiring to an overload when the expected and actual are the same type.

The implementation kept them distinct from the existing non-generic versions in order to avoid lots of internal rework but caused innumerable overload resolution conflicts for the compiler.