Drop pre-conversion of test case/fixture params where .NET supports it implicitly

[stevenaw]

NUnit has historically handled and pre-converted some type conversions when passing parameters to test cases/fixtures. These include both native-supported implicit conversions of primitives as per the ECMA spec (ex: int -> long or short -> int) and some special case conversions for ease-of-use (like string -> DateTime). Preconversion of primitives within NUnit of types also implicitly converted by netfx + netcore has also meant some bugs and added complexity:

• TestCaseSource "Single" cannot be converted to "int" issue #3125
• ValuesAttribute uses an outdated version of TestCaseAttribute's type coercion logic #2638
• Consolidate type conversion code  #3219

I think it should be considered to drop this preconversion of NUnit-provided ECMA-defined implicit conversions. This would not include removing ease-of-use custom conversions like string -> DateTime which should make it a relatively transparent change to consumers. One point of discussion is that this preconversion was originally added for Compact Framework, and that removing it could disrupt any CF consumers (if that is still supported)

I'd like to hear some thoughts on this.

[stevenaw]

A recent pull request (#4307 ) for a bugfix caused me to think back to this issue.
@nunit/framework-team @nunit/core-team @oznetmaster Is there interest in this issue proceeding or should it be closed?