[Java.Base, generator] Bind all of package java.io

[jonpryor]

Context: bc5bcf4

Bind all classes and interfaces in the java.io package.

Binding the java.io.StringBufferInputStream.buffer field
exposed a generator bug: property setters for String fields would
not compile, producing:

partial class StringBufferInputStream {
    protected string? Buffer {
        get { ... }
        set {
            const string __id = "buffer.Ljava/lang/String;";

            var native_value = global::Java.Interop.JniEnvironment.Strings.NewString (value);
            try {
                _members.InstanceFields.SetValue (__id, this, native_value?.PeerReference ?? default);
            } finally {
                global::Java.Interop.JniObjectReference.Dispose (ref native_value);
                GC.KeepAlive (value);
            }
        }
    }
}

Specifically, native_value?.PeerReference is not valid, and
unnecessary. This was due to a bug in BoundFieldAsProperty.cs,
as String field marshaling hit the "Object" marshaling path, which
was not warranted in this case, as String is treated specially.

Fix this bug.

Binding [java.io.ObjectOutputStream.PutField.write(ObjectOutput)][2]
presented a larger binding problem: it's an abstract method on an
abstract class, but the PutFieldInvoker.Write() method was
[Obsolete] when PutField.Write() was not!

public partial class /* ObjectOutputStream. */ PutField {
    public abstract void Write (Java.IO.IObjectOutput? p0);
}
internal partial class /* ObjectOutputStream. */ PutFieldInvoker : PutField {
    [Obsolete (@"deprecated")]
    public override void Write (Java.IO.IObjectOutput? p0) => ...
}

This state of affairs triggers a CS0809 warning, which is an error in
Release configuration builds:

##[error]src/Java.Base/obj/Release-net6.0/mcw/Java.IO.ObjectOutputStream.cs(311,32)
Error CS0809: Obsolete member 'ObjectOutputStream.PutFieldInvoker.Write(IObjectOutput?)' overrides non-obsolete member 'ObjectOutputStream.PutField.Write(IObjectOutput?)'

Turns Out that BoundMethodAbstractDeclaration.cs never "forwarded"
deprecated attributes? Meaning that abstract methods would never
be [Obsolete], even if their Java peer was @deprecated?

Update BoundMethodAbstractDeclaration so that if the Java method is
deprecated, the bound abstract method is [Obsolete]. This fixes
the CS0809, as now PutField.Write() and PutFieldInvoker.Write()
are consistent with each other.

TODO: should this change be done for all codegen targets?

Finally, in order to make it easier to view and review the
current Java.Base.dll API -- without requiring that it be built
locally -- add a _GenerateRefApi target to Java.Base.targets
which uses the Microsoft.DotNet.GenAPI NuGet package to generate
"reference assembly source" for Java.Base.dll. This is stored in
src\Java.Base-ref.cs.

[jpobst]

Seems like it would make sense to always prevent that warning. In the real world it's nearly impossible to bind libraries with <TreatWarningsAsErrors>, but it's something we should continue working towards.

[jonpryor]

@jpobst: The implicit question here is whether all CodeGenerationTargets should add [Obsolete] to abstract methods, or just CodeGenerationTarget.JavaInterop1.

I opted to only update CodeGenerationTarget.JavaInterop1 for expediency, but I suspect this should be done for everything.