Implement I2CResource

[awygle]

This PR adds an I2CResource in the same vein as SPIResource et al.

The I/O standards required for I2C do not change depending on whether the device acts as peripheral or controller, due to the ability of peripherals to stretch the clock by driving SCL. Therefore I don't bother to take in a role argument like SPIResource does.

I added the *_pullup arguments after the fashion of DirectI2CResource because the Arty A7 board file was using that construct.

I did not modify the Atlys (I2C inside of HDMI) or the Arty Z7 (I2C inside a Connector) board definitions, but did update all the other boards I found using Resource for I2C.

[awygle]

Per IRC discussion, removed the pullup pins. We can add them later if someone needs them.

[whitequark]

Thanks!