WIP: Add Caller/Callers fields

[jcorbin]

This PR is essentially a proposal-as-prototype-code, rather than than proposal-as-issue; how about:

• replacing zap.Stack with zap.Callers
• simplifying the zap.AddCaller hook to just add a zap.Caller field, rather than mutate msg...
• ...oh and also let the user pass a skip offset to zap.AddCaller so that it can get it right under logger wrappers

Other questions include:

• what if we added proper array support, rather than the hacked object? (see also JSON Array support #136)
• maybe we should have explicit PC(s) marshaling support in the the KeyValue interface itself...
• ...this would let the text encoder recover a more human friendly stacktrace-esque form
• ...maybe the Error field should also always (modulo an Option?) add a callers field?

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[jcorbin]

Superceded by #201, will open an issue to discuss what missing functionality we want to support going forward.