This partially fixes #884.

This implementation is limited to top level fields, like:

    Int64("int64", 123)
    Float64("float64", 456)

Or sugared:

    With("int64", 123, "float64", float64(456), "slice", []int{1, 2, 3})

It does NOT work with reflected fields like:

    Reflect("slice", []int{1, 2, 3})
    With("struct", struct{ Int64 int64 }{Int64: 123})

As for reflected fields we use go's stdlib json encoder directly, which
doesn't provide such feature.

Just doing

    go mod edit --replace=go.uber.org/zap=github.com/reddit/[email protected]

In Baseplate.go won't cause all the go.mod files in services importing
Baseplate.go to do the same replacement, so although it works for
Baseplate.go, the service code won't build.

We have to change all the import path inside this repo into
github.com/reddit/zap so that we can change the import from
go.uber.org/zap into github.com/reddit/zap in Baseplate.go.