Miscellaneous cleanups for HAL, wpilibc, and wpilibj JNI (#589)

* Static functions in the HAL implementation were placed in the hal namespace
* "using namespace" declarations in HAL/cpp/Log.h and Timer.cpp were replaced
  with "using" declarations for std::chrono
* An extra include was removed from AnalogGyro.cpp
* InterruptableSensorBase's constructor was defaulted
* Newlines were added to some wpilibc integration tests for grouping
* A variable in HALUtil.h was renamed to follow the style guide

Supersedes #586

hal/lib/athena/Power.cpp

@@ -13,6 +13,8 @@
 
 using namespace hal;
 
+namespace hal {
+
 static std::unique_ptr<tPower> power;
 
 static void initializePower(int32_t* status) {
@@ -21,6 +23,8 @@ static void initializePower(int32_t* status) {
   }
 }
 
+}  // namespace hal
+
 extern "C" {
 
 /**