Button circuit prototype

[rrbutani]

Before we manufacture board V0, we should build and test the ADC/digital in button circuit.

Since we're mostly concerned about the electrical soundness of the design, we can test this on a breadboard with a TM4C.

We should check that:

• [x] each button yields a different ADC reading (the ESP32 and the TM4C both have 12-bit ADCs but really we shouldn't need nearly that much resolution for this)
• [x] all combinations of the five buttons yield discernible readings (not required but this should be possible if the design works)
• [x] single or multiple buttons being pressed result in the digital input going high
• [x] the digital input is adequately de-bounced
• [x] the circuit never draws excessive amounts of current (<1mA)

[rrbutani]

Another thing to think about (for later) is whether we should poll the ADC constantly (or repeatedly, at least) while the digital input is high.

The edge case to consider is when a button is pressed and, before that button is released, another button is pressed. The second button press (and the release of one of the two buttons) will not trigger interrupts which means that if we don't poll the ADC on a timer while the digital input is high, we'll miss such button presses.

This isn't necessarily bad; ignoring button presses while another button is already held might be perfectly reasonable behavior.

[rrbutani]

Finished by @spendharkar in 041b4806ed6dc4f6f6d12590575530fffd11401e.