Field internals such as FieldType should not be exported

[prashantv]

I think this changed as part of the zap/zapcore split, but the original plan with fields was to keep the implementation details completely private, and the only way to use a field would be to AddTo(Encoder).

It seems like FieldType is exported only to let the zap package expose field constructors, but I think what we should unexport all implementation details and instead:

• Expose the same field constructors in the zapcore package
• The zap package just wraps the underlying zapcore functions, and doesn't need access to the internals of a field.

This gives us more flexibility to change Field implementation details in future.

It's a little late for 1.0, but perhaps in a far away 2.0.

Thoughts? @akshayjshah @billf

[akshayjshah]

I'll add this to a wishlist issue for a future major version release; we can discuss details then.

For the 1.0, I was weighing this implementation detail leak against the discoverability/documentation confusion cost of duplicating the field constructors. If we're going to keep fields completely opaque, we should just collapse everything back into one package - there are waaaaay too many field constructors (even for just the basic types) to make duplicating them worthwhile.

[akshayjshah]

Moved to #388.