encoding/yaml: eliminate use of ghodss yaml pkg

This implementation translates a CUE AST to yaml.Node
for the yaml.v3 package.

The new builtins now also verify the concreteness of
the value.  Closes #288.

Also, the extra roundtrip by ghodss translates CUE to
a map, which then causes the fields to be ordered
lexically, which is undesirable.
The new encoder leaves fields in CUE ordering
which is closer to the original ordering of the
fields. This causes large (desirable) changes in the
Kubernetes' demo validation data.

Change-Id: Iedd32c46b9738d9c367628087c918c4c70310e1d
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5087
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/go.sum b/go.sum
index aa4e3c4..5537cf7 100644
--- a/go.sum
+++ b/go.sum
@@ -6,8 +6,6 @@
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/emicklei/proto v1.6.15 h1:XbpwxmuOPrdES97FrSfpyy67SSCV/wBIKXqgJzh6hNw=
 github.com/emicklei/proto v1.6.15/go.mod h1:rn1FgRS/FANiZdD2djyH7TMA9jdRDcYQ9IEN9yvjX0A=
-github.com/ghodss/yaml v1.0.0 h1:wQHKEahhL6wmXdzwWG11gIVCkOv05bNOh+Rxn0yngAk=
-github.com/ghodss/yaml v1.0.0/go.mod h1:4dBDuWmgqj2HViK6kFavaiC9ZROes6MMH2rRYeMEF04=
 github.com/google/go-cmp v0.4.0 h1:xsAVV57WRhGj6kEIi8ReJzQlHHqcBYCElAvkovg3B/4=
 github.com/google/go-cmp v0.4.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/inconshreveable/mousetrap v1.0.0 h1:Z8tu5sraLXCXIcARxBp/8cbvlwVa7Z1NHg9XEKhtSvM=
@@ -61,5 +59,5 @@
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/errgo.v2 v2.1.0 h1:0vLT13EuvQ0hNvakwLuFZ/jYrLp5F3kcWHXdRggjCE8=
 gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
-gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
-gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v3 v3.0.0-20200121175148-a6ecf24a6d71 h1:Xe2gvTZUJpsvOWUnvmL/tmhVBZUmHSvLbMjRj6NUUKo=
+gopkg.in/yaml.v3 v3.0.0-20200121175148-a6ecf24a6d71/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=