internal/third_party/yaml: parse into CUE ast

Also:
- remove support for parsing into Go-structs
- remove support for encoding
- add comment parsing
- port non-standard test harness to standard go
- add testdata
- fix some bugs related to merging

Change-Id: I7fbd2bed6f76bb5da8a309f8d3fb3fb6228048ab
diff --git a/internal/third_party/yaml/testdata/merge.test b/internal/third_party/yaml/testdata/merge.test
new file mode 100644
index 0000000..733ba2b
--- /dev/null
+++ b/internal/third_party/yaml/testdata/merge.test
@@ -0,0 +1,54 @@
+# From http://yaml.org/type/merge.html
+# Test
+anchors:
+  list:
+    - &CENTER { "x": 1, "y": 2 }
+    - &LEFT   { "x": 0, "y": 2 }
+    - &BIG    { "r": 10 }
+    - &SMALL  { "r": 1 }
+
+# All the following maps are equal:
+
+plain:
+  # Explicit keys
+  "x": 1
+  "y": 2
+  "r": 10
+  label: center/big
+
+mergeOne:
+  # Merge one map
+  << : *CENTER
+  "r": 10
+  label: center/big
+
+mergeMultiple:
+  # Merge multiple maps
+  << : [ *CENTER, *BIG ]
+  label: center/big
+
+override:
+  # Override
+  << : [ *BIG, *LEFT, *SMALL ]
+  "x": 1
+  label: center/big
+
+shortTag:
+  # Explicit short merge tag
+  !!merge "<<" : [ *CENTER, *BIG ]
+  label: center/big
+
+longTag:
+  # Explicit merge long tag
+  !<tag:yaml.org,2002:merge> "<<" : [ *CENTER, *BIG ]
+  label: center/big
+
+inlineMap:
+  # Inlined map
+  << : {"x": 1, "y": 2, "r": 10}
+  label: center/big
+
+inlineSequenceMap:
+  # Inlined map in sequence
+  << : [ *CENTER, {"r": 10} ]
+  label: center/big
\ No newline at end of file