Aliases that ruyaml thinks are unreferenced seem to be dropped

[gfokkema]

I'm noticing some surprising behaviour where ruyaml seems to drop aliases that it thinks are not referenced.
I think this causes issues when dealing with duplicate merge keys as described in #43 for example.

Using this sample function to spot differences between input and output:

yaml = ruyaml.YAML(typ='rt')
yaml.allow_duplicate_keys = True
def read(srcdata):
    with io.StringIO() as buf:
        yaml.dump(yaml.load(srcdata), buf)
        dstdata = buf.getvalue()
    sys.stdout.writelines(difflib.unified_diff(srcdata.splitlines(True), dstdata.splitlines(True)))

When using the "correct" merge syntax as mentioned in #43:

read('''\
testa: &test1
  a: 1
testb: &test2
  a: 2
testc: &test3
  a: 3

data:
  <<: [*test1, *test2]
  c: 3
''')

I get these results:

--- 
+++ 
@@ -2,7 +2,7 @@
   a: 1
 testb: &test2
   a: 2
-testc: &test3
+testc:
   a: 3
 
 data:

And when using the "incorrect" syntax, things become more surprising:

read('''\
testa: &test1
  a: 1
testb: &test2
  a: 2
testc: &test3
  a: 3

data:
  <<: *test1
  <<: *test2
  <<: *test3
  c: 3
''')

I get these results:

--- 
+++ 
@@ -1,12 +1,11 @@
 testa: &test1
   a: 1
-testb: &test2
+testb:
   a: 2
 testc: &test3
   a: 3
 
 data:
   <<: *test1
-  <<: *test2
   <<: *test3
   c: 3

Any ideas what could be the cause or suggestions on how to deal with this?