cue: add optional fields

Optional fields are crucial in making it manageable
to work with very large struct types, like those of
Kubernetes. Without optional fields, the fields of
a simple K8s configuration  would be lost in the noise
compared to the default value of every expanded field.

It also allows writing:
  foo?: MyStruct
instead of
  foo: *null | MyStruct
which gets old quickly when one has hundreds of
such fields.

Updates #24

Change-Id: I856d9a3e20584750b911784924fa18d4557b9920
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1700
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/go.sum b/go.sum
index 40485fd..3d202df 100644
--- a/go.sum
+++ b/go.sum
@@ -1,9 +1,12 @@
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+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
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@@ -13,6 +16,10 @@
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+github.com/inconshreveable/mousetrap v1.0.0 h1:Z8tu5sraLXCXIcARxBp/8cbvlwVa7Z1NHg9XEKhtSvM=
+github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
+github.com/lib/pq v1.0.0 h1:X5PMW56eZitiTeO7tKzZxFCSpbFZJtkMMooicw2us9A=
+github.com/lib/pq v1.0.0/go.mod h1:5WUZQaWbwv1U+lTReE5YruASi9Al49XbQIvNi/34Woo=
 github.com/magiconair/properties v1.8.0 h1:LLgXmsheXeRoUOBOjtwPQCWIYqM/LU1ayDtDePerRcY=
 github.com/magiconair/properties v1.8.0/go.mod h1:PppfXfuXeibc/6YijjN8zIbojt8czPbwD3XqdrwzmxQ=
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@@ -23,6 +30,7 @@
 github.com/pelletier/go-toml v1.2.0/go.mod h1:5z9KED0ma1S8pY6P1sdut58dfprrGBbd/94hg7ilaic=
 github.com/pkg/errors v0.8.0 h1:WdK/asTD0HN+q6hsWO3/vpuAkAr+tw6aNJNDFFf0+qw=
 github.com/pkg/errors v0.8.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
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 github.com/spf13/afero v1.1.2/go.mod h1:j4pytiNVoe2o6bmDsKpLACNPDBIoEAkihy7loJ1B0CQ=
@@ -36,14 +44,11 @@
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-golang.org/x/exp/errors v0.0.0-20181210123644-7d6377eee41f h1:B/8yFg7PHSFdahc+fMB+RUy3if9GlZmexAbcdfCwREI=
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-golang.org/x/exp/errors v0.0.0-20181220081853-a8d4f384862a h1:juhXrq7Jv3Yw6H4azsxx0SGft00IiERvkUW6cNqnV1I=
-golang.org/x/exp/errors v0.0.0-20181220081853-a8d4f384862a/go.mod h1:YgqsNsAu4fTvlab/7uiYK9LJrCIzKg/NiZUIH1/ayqo=
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@@ -54,6 +59,7 @@
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