Exclude primitive types when comparing by members

[jnyrup]

As primitive types do not have members, it does not make sense to try to compare them by members.
On all but one of the target frameworks two different primitives are currently incorrectly evaluated as being equivalent (#1374).
On net472 it will even hit "The maximum recursion depth was reached".

This PR excludes primitive types from being compared by members.

Thought:
Should we add a guard check in ComparingByMembers if someone writes
ComparingByMembers<int>() and wonders why ints are still not compared by members?

For those interested in the complete set of primitive types, here are the facts from in .netcoreapp3.0

AllTypes
  .From(typeof(int).Assembly)
  .Where(e => e.IsPrimitive)
  .Select(e => e.Name)
  .ToList();

"Boolean"
"Byte"
"Char"
"Double"
"Int16"
"Int32"
"Int64"
"IntPtr"
"SByte"
"Single"
"UInt16"
"UInt32"
"UInt64"
"UIntPtr"

[dennisdoomen]

Should we add a guard check in ComparingByMembers if someone writes
ComparingByMembers() and wonders why ints are still not compared by members?

Yes, I think it should throw an InvalidOperationException

[dennisdoomen]

Don't we need some update to the docs and release notes?

[jnyrup]

🤦 Completely forgot about docs and release notes .

[dennisdoomen]

What about https://fluentassertions.com/objectgraphs/#value-types?

[jnyrup]

Added notes to the object graphs section.