platforms/silicon: remove usage of lib.io.Pin (deprecated).

[jfng]

Instead, SiliconPlatform.request() returns an instance of SiliconPlatformPort.

Fixes #19.

[whitequark]

I'm confused about this PR. The SiliconPlatformPort looks kind of like it's intended to be a lib.io.PortLike, but it's not, and doesn't enable using it with lib.io.Buffer. Any new cores are going to use lib.io.Buffer so this PR isn't sufficient.

If the goal is to just remove the deprecation, lib.io.Buffer.Signature(...).create() is sufficient to replace Pin(...).

[jfng]

I'm confused about this PR. The SiliconPlatformPort looks kind of like it's intended to be a lib.io.PortLike, but it's not, and doesn't enable using it with lib.io.Buffer. Any new cores are going to use lib.io.Buffer so this PR isn't sufficient.

It wasn't a PortLike because it had no support for __invert__(), and even if it was, it wouldn't be usable with lib.io buffers without a significant rewrite of the platform.

If the goal is to just remove the deprecation, lib.io.Buffer.Signature(...).create() is sufficient to replace Pin(...).

I updated the PR to return an interface created from lib.io.Buffer.Signature.

[whitequark]

It wasn't a PortLike because it had no support for __invert__(), and even if it was, it wouldn't be usable with lib.io buffers without a significant rewrite of the platform.

Right. I recall that the rewrite of the platform was planned on one of our meetings, which is why I was confused. This PR works fine as a backstop until then.

[whitequark]

LGTM, thanks!