Add and use 'string_to_fraction()' helper converter function

This matches the pattern of unit conversion, and will allow us to remove
all the code that uses the old complex "CHANGED()" macro that tries to
remove units or percent signs.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

dive.h

@@ -820,6 +820,7 @@ extern weight_t string_to_weight(const char *str);
 extern depth_t string_to_depth(const char *str);
 extern pressure_t string_to_pressure(const char *str);
 extern volume_t string_to_volume(const char *str, pressure_t workp);
+extern fraction_t string_to_fraction(const char *str);
 
 #include "pref.h"