[ios, osx] Split MGLDirectionFormatter in two

Split MGLDirectionFormatter into MGLClockDirectionFormatter, which specializes in clock positions, and MGLCompassDirectionFormatter, which boxes the compass. This neatly avoids any confusion that might arise if the developer fails to read the documentation and thinks the original unified formatter can convert between the two types of directions.

platform/darwin/include/MGLClockDirectionFormatter.h

@@ -0,0 +1,52 @@
+#import <Foundation/Foundation.h>
+#import <CoreLocation/CoreLocation.h>
+
+#import "MGLTypes.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ The `MGLClockDirectionFormatter` class provides properly formatted descriptions
+ of headings relative to the user, known as <i>clock positions</i>. For
+ example, a value of `90` may be formatted as “3 o’clock”, depending on the
+ locale.
+ 
+ Use this class to create localized heading strings when displaying directions
+ relative to the user’s current location and heading. To format a direction
+ irrespective of the user’s orientation, use `MGLCompassDirectionFormatter`
+ instead.
+ */
+@interface MGLClockDirectionFormatter : NSFormatter
+
+/**
+ The unit style used by this formatter.
+ 
+ This property defaults to `NSFormattingUnitStyleMedium`.
+ */
+@property (nonatomic) NSFormattingUnitStyle unitStyle;
+
+/**
+ The locale of the receiver.
+ 
+ The locale determines the output language and numeral system of the output.
+ */
+@property (copy) NSLocale *locale;
+
+/**
+ Returns a clock position string for the provided value.
+ 
+ @param direction The heading, measured in degrees, where 0° means “straight
+    ahead” and 90° means “directly to your right”.
+ @return The clock position string appropriately formatted for the receiver’s
+    locale.
+ */
+- (NSString *)stringFromDirection:(CLLocationDirection)direction;
+
+/**
+ This method is not supported for the `MGLDirectionFormatter` class.
+ */
+- (BOOL)getObjectValue:(out id __nullable * __nullable)obj forString:(NSString *)string errorDescription:(out NSString * __nullable * __nullable)error;
+
+@end
+
+NS_ASSUME_NONNULL_END