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/readerc.go b/internal/third_party/yaml/readerc.go
index 7c1f5fa..b0c436c 100644
--- a/internal/third_party/yaml/readerc.go
+++ b/internal/third_party/yaml/readerc.go
@@ -95,7 +95,7 @@
 
 	// [Go] This function was changed to guarantee the requested length size at EOF.
 	// The fact we need to do this is pretty awful, but the description above implies
-	// for that to be the case, and there are tests 
+	// for that to be the case, and there are tests
 
 	// If the EOF flag is set and the raw buffer is empty, do nothing.
 	if parser.eof && parser.raw_buffer_pos == len(parser.raw_buffer) {