Split HAL Digital Implementation files (#59)

Split to match the new headers. Uses a namespace 'hal' for internal functions and globals.
SPIAccumulator merged back into SPI header, as it was not a good split. Analog
accumulator will move back to analog input when the analog split is done.

hal/lib/athena/Interrupts.cpp

@@ -9,8 +9,9 @@
 
 #include "ChipObject.h"
 
-extern void remapDigitalSource(bool analogTrigger, uint32_t& pin,
-                               uint8_t& module);
+#include "DigitalInternal.h"
+
+using namespace hal;
 
 struct Interrupt  // FIXME: why is this internal?
 {