Handle analogRead(n) for n in 8..15

[WestfW]

Let's try this again...

The code in analogRead() does not not handle integer arguments of 8..15, which are valid analog pins on Grand Central. Worse, it will do "bad things", like setting PINMUX to "analog" on pins that don't even have analog support.
#236

[WestfW]

This "bug" exists on other platforms with more than 8 analog inputs as well, since the analogRead() code only adds the offset for n<=5, n=6, and n=7.
On all of them, A0..An are contiguous, so it doesn't NEED the (p-8)+A8 math. But it won't hurt, either. (and actually, it gets eliminated by the optimizer.)

[WestfW]

Edit: Never mind. It was my own sketch that was complaining about an undefined PIN_A8.
I've confirmed that the changed version (as per the PR) compiles on all of the Adafruit SAMD platforms...

Hmm. feather m4 doesn't compile. It has NUM_ANALOG_INPUTS less than 8, so it shouldn't invoke the code that it's complaining about.
Does the preprocessor handle #if 6u > 8 correctly?
I don't understand why NUM_ANALOG_INPUTS has to be explicitly unsigned in the first place?

[ladyada]

is this ready for review?

[WestfW]

It WAS ready, and I'm confident that it works for GCM4 and such, but I've recently become a bit worried that it might not work on all the new platforms with >8 analog inputs :-(