grow /exp/zapfield

[mikhalytch]

adds following generic Field constructors:

• Uint
• Uint64
• Uint32
• Uint16
• Uint8
• Int
• Int64
• Int32
• Int16
• Int8
• Float64
• Float32
• String
• Strings

[CLAassistant]

All committers have signed the CLA.

[abhinav]

Hey, @mikhalytch, thanks for this PR, and apologies for the delay.

I'd like to better understand the motivation behind this proposal.
What problem are we trying to solve with these type-parameterized field constructors?
Is this another version of the problem from #1275?

[abhinav]

Separately, note that while func Str and func Strs had the possibility of being moved to the top-level Zap package, func String, func Strings, and these functions do not (because of the conflict). That's not the end of the world, but it means that if we ever consider these APIs for promotion to the main Zap module, they'll still have to be under a zapfield subpackage.