(From Jon:) Currently, FixedDateTimeZone overrides Equals(object) to compare 
itself with other fixed zones, but other time zones don't.  (Malcolm: This is 
bad for e.g. ZonedDateTime's Equals(object) implementation.)

It would be useful for zones to implement equality sensibly. In particular, 
asking a single IDateTimeZoneSource for a zone with the same ID should yield 
references to equal objects, even if they're two separate objects.

Whether two zones from *separate* sources which share the same original source 
data are equal is probably best left to be source-specific, although it would 
be preferable if they were.

(Malcolm: See also issue 65 for some more of the above.)

If we make DateTimeZone implement IEquatable<DateTimeZone> but with an abstract 
Equals(DateTimeZone) method, this will force implementations to at least 
acknowledge that they ought to be doing something sensible. It can also 
override Equals(object) to call Equals(DateTimeZone) appropriately so this can 
be done in just one place.