Support deserialization of YAML anchors of value types

[HertzDevil]

Resolves #13830. All objects are now boxed so that they can be stored into the parse context uniformly.

This does not affect how the GC sees YAML::ParseContext#@anchors; since the hash's key type is String, Hash::Entry(String, T) is always non-atomic, thus the switch from UInt64 to Void* makes no differences. (In fact Void* is probably more correct here.)

YAML.parse already handles anchors regardless of their types, since they are all YAML::Anys; YAML::Nodes.parse returns a document and never resolves aliases. These methods are not affected in this patch.

[Sija]

Would be a nice company to a #15941 given the slew of serialization-related changes in 1.18.0... ^^

[straight-shoota]

We're past the merge window for 1.18, so this has to wait for 1.19.