Add APDU debugging using a trace api similar to httptrace

[areese]

@ericchiang Please let me know what you think of this way of tracing.
I need to sign the commits, I don't have the keys right now, and I need to try this out, I don't have the harness.

[ericchiang]

Rather than using contexts, I would recommend the exported API look something:

type ClientTrace struct {
    Transmit func(req []byte)
    TransmitResult func(...)
}
type Client struct {
    Trace *ClientTrace
}
func (c *Client) Open(*Yubikey, error) { ... }

Dropping the WithClientTrace methods and use of contexts

[areese]

Rather than using contexts, I would recommend the exported API look something:

type ClientTrace struct {
    Transmit func(req []byte)
    TransmitResult func(...)
}
type Client struct {
    Trace *ClientTrace
}
func (c *Client) Open(*Yubikey, error) { ... }

Dropping the WithClientTrace methods and use of contexts

I think that's way better. I didn't think of that path, and just added context.