Support serializing and deserializing atomic objects to/from JSON using
the JSON representation of the underlying types. Without this,
marshaling returns `{}`.
Per discussion in #68, `atomic.Value` does not yet implement support
because there's an open question as to whether it should implement it
even if the underlying type doesn't support JSON marshaling/
unmarshaling.
Resolves#49
This optimization causes data races since we're changing the value field
without using atomics. E.g., a caller who has multiple goroutines
calling `Set("1")` and `Set("")` will race on the access to `s.v` since
one goroutine is trying to read it while the other sets it, neither
using atomic operations.
This reverts commit 16b44f14f0.
