[examples f1] ADC+DAC very basic example

Just shows some basic working examples of using the DAC with the ADC. No
special features, but very little hardware required to demonstrate.
[adc common] move f1 and l1 to "common"

Leaves f2 and f4 off for now, while this gets tested out.  Compiles at
least, but needs some more work to verify.

L1 and F0 ref manuals all talk about "ADC" but F1,F2,F3,F4 all talk
about ADC1 (even on devices that only have 1)  I've added aliases for
the L1 to fake this out.

One note, I removed runtime checks that were aimed at preventing
enabling the DMA bit (ADC_CR2) for peripherals that don't support it.
I didn't think it was worthwhile to add #if #defines there when the bit
is simply ignored if it has no impact.
[l1+examples] working adc+dac, but rough api

This uses the "unified" adc code for both l1 and f1
[examples f1] port to unified adc code
[adc l1/f1] Make some more code standard

Fix the sequencing to work for single and multi channel, and improve the
documentation to go with it.
[l1] Include missing High density definition