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.
Unexported field name means the methods on different types look
more similar (e.g., &i.v instead of &i.int32 or &i.uint32).
For String, this is important since we don't want users to be
able to access the underlying atomic.Value
