Merge #879

879: Improve ADC configuration options r=Dirbaio a=chemicstry

This smooths out a few rough edges with ADC:
- Make `Resolution::to_max_count()` pub for all ADC versions. Useful if performing conversion manually.
- `VREF` and `VREFINT` were convoluted. `VREF` is an external voltage reference used for all ADC conversions and it is exposed as a separate pin on large chips, while on smaller ones it is connected to `VDDA` internally. `VREFINT` is an internal voltage reference connected to one of the ADC channels and can be used to calculate unknown `VREF` voltage.
- Add a separate `VREF_DEFAULT_MV`, equal to 3.3V, which is the usual supply voltage and should work for most simple cases.
  - For an external voltage reference `set_vref()` can be used to set a known precise voltage.
  - If no voltage reference is present, `VREFINT` calibration can be used to calculate unknown `VREF` voltage. If I understand correctly, this is only implemented for `adc_v3` (L4?), but is not currently exposed (private). If someone is interested, this can be fixed in separate PR.

Co-authored-by: chemicstry <chemicstry@gmail.com>

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