Digital single-source refactor and abstract comparator

[ducky64]

Merges DigitalSingleSource into DigitalSource, plus a bunch of associated changes. Infrastructure to support an AbstractComparator class, which can have open-drain and push-pull outputs. Resolves #387

Core compiler changes:

• Sum(BoolArray) does a counting operation. Add Vector.count(elt => BoolExpr) and ArrayExpr.count() in the frontend.
• More robust NaN (RangeEmpty) propagation to reduce compiler crashes

Model and part changes:

• DigitalSource now supports separate high / low (open-drain) / push-pull / no driver (pullup / down) ports. Some of the reasons why DigitalSingleSource was necessary before are obsoleted by introducing the concept of bridged internal ports, which have some checks disabled (here, presence of a signal driver) which could resolved by the upper-level link.
• Support DigitalSink with internal pullup / pulldown (as is common on reset pins)
• Improve I2C bridge parameter propagation when pullup is not connected (eg, in an inner link)
• Change SWD and ESP32 reset ports to use DigitalSource pulldown - kind of a hack, but a bit cleaner than before
• Change chip reset modeling to use DigitalSink with pullup
• Generate netlists for blinky examples