Introduce basic context.Context compatibility

[tsenart]

This change set introduces basic context.Context compatibility.

Specifically:

• Adds a ConnectContext method to Conn and defines Connect in terms of it.
• Adds a CallContext method to Conn and defines Call in terms of it.
• Adds a PerformCallContext function and defines PerformCall in terms of it.
• Adds a DialContext method to Dialer and defines Dial in terms of it.
• Adds a DialContext function and defines Dial in terms of it.

Higher level utility methods such as ManifestCreateTask are left unchanged.

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