pkg/encoding/json: Unmarshal() gives mangled result

[cueckoo]

Originally opened by @enisoc in cuelang/cue#21

As of current master, the following CUE program doesn't evaluate as expected:

import "encoding/json"

input: "{\"one\": 1, \"two\": 2, \"three\": 3}"
result: json.Unmarshal(input)

Expected output:

{
    "input": "{\"one\": 1, \"two\": 2, \"three\": 3}",
    "result": {
        "one": 1,
        "two": 2,
        "three": 3
    }
}

Actual output (cue export):

{
    "input": "{\"one\": 1, \"two\": 2, \"three\": 3}",
    "result": {
        "Valid": 1,
        "Compact": 2,
        "Indent": 3
    }
}

Notice the field names in the unmarshaled result have been replaced with the names of methods in pkg/encoding/json.

[cueckoo]

Original reply by @mpvl in cuelang/cue#21 (comment)

fixed