Add KeyValueMap, a KeyValue implemented backed by a map

[prashantv]

This is a useful utility when testing custom MarhsalLog implementations

cc @billf: Can you make sure that this works for you to use in #85

[akshayjshah]

What do you think about putting this in its own package? Once we clean up and export the encoder type, we'll want this to implement the encoder interface, and we'll probably want to use it in some tests, in zwrap, and in zbark.

[prashantv]

Yep, I thought it belonged to another package, but wasn't sure what that package should be.

I considered zencoder but don't really think that's right. Any suggestions?

[akshayjshah]

How about mapencoder? mapencoder.New sounds like a reasonable constructor.

[prashantv]

Hmm I'm not sure about encoder in the name since it's an implementation of the KeyValue interface. The interface abstracts the JSON encoder and encoding in general, but the KeyValueMap doesn't really fulfill a full encoder, it only handles the fields.

However, I don't have strong opinions on this so if we can't come up with anything better, then mapencoder is OK.

[akshayjshah]

lgtm. let's drop it wherever makes sense, maybe in the core zap package.

[prashantv]

Leaving it in zwrap for now, we can move it in to core later as we make changes for 1.0