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/apic.go b/internal/third_party/yaml/apic.go
index 1f7e87e..9cf9005 100644
--- a/internal/third_party/yaml/apic.go
+++ b/internal/third_party/yaml/apic.go
@@ -24,8 +24,9 @@
 }
 
 // Create a new parser object.
-func yaml_parser_initialize(parser *yaml_parser_t) bool {
+func yaml_parser_initialize(parser *yaml_parser_t, filename string) bool {
 	*parser = yaml_parser_t{
+		filename:   filename,
 		raw_buffer: make([]byte, 0, input_raw_buffer_size),
 		buffer:     make([]byte, 0, input_buffer_size),
 	}