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Support empty keys in block and flow mappings (#558)

* Support empty keys in block and flow mappings

An empty key was a special case at the document root only, where it produced
an entry with an empty string key. Everywhere else a key separator without a
preceding key was rejected, and in a flow mapping it was ignored, which
dropped the entry:

```yaml
key: value
: empty key      # parse_error: Detected invalid indentation
- : foo          # parse_error: mapping key should not be empty
{ key: value, : empty key }
                 # parse_error: The ":" mapping value indicator must be
                 # followed after a mapping key
```

Parse an empty key as a null node, as the YAML specification does, and add the
entry wherever a key separator is found with no key before it: at the start of
a line in a block mapping, in a block sequence entry, and in a flow mapping.
The value of an empty key can be omitted as well, so add_empty_key_entry()
also moves back to the parent mapping when the following token is not indented
deeper, the same way an omitted value is handled for a normal key.

This changes the type of an empty key from an empty string to a null node,
which is a breaking change for anyone reading such an entry as node[""].
A quoted empty key ("": value) is unaffected and remains a string.

Spec example 8.18 (test suite case S3PD) and the empty key cases NKF9 and PW8X
of the yaml-test-suite now parse. CFD4 (an empty key in a single pair flow
sequence) is still rejected: it needs single pair flow mappings, which are not
supported for non-empty keys either ([foo: 1] is rejected as well).

* Parse a key separator which begins the contents of an explicit key

The contents of an explicit key may themselves be a mapping entry with an
empty key, as pointed out in #557: `? ` introduces an explicit key, and the
`:` which follows it on the same line belongs to that key rather than to the
entry the key is part of.

Handle a key separator which begins the ... (continued)

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