On Mono, not all CLR members defined on int are available on all int instances

Python type int is backed by BigInteger, but both BigInteger and Int32 instances are used as int instances for performance reasons (#52). For the Python type system consistency, any attributes available on int (thus BigInteger) should be available on instances of int (thus also on Int32 instances). PR #1399 implemented a set of properties and methods that are available on BigInteger but not on Int32 as additional extension methods for Int32. However, this does not work well on Mono (v6.12.0.162 on darwin, but possibly on linux too). The following methods appear on int that do not appear on Int32 instances:

• GetByteCount
• TryWriteBytes

These two methods do not exist in .NET Framework 4.6 so they should not exist in Mono as well. And indeed, they do not, at least not as members of System.Numerics.BigInteger (so they are not accessible in C# code). However, somehow they do appear in IronPython, perhaps they are implemented as extensions?

Start by reproducing the member differences between BigInteger and Int32 on Mono 6.12.0.162, then inspect PR #1399 and the extension methods it added. Done means GetByteCount and TryWriteBytes have consistent availability on int instances without exposing members absent from the target .NET Framework.