Context: bc5bcf4

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

Binding the [`java.io.StringBufferInputStream.buffer`][0] 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`.

[0]: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/StringBufferInputStream.html#buffer
[1]: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/ObjectOutputStream.PutField.html#write(java.io.ObjectOutput)