Change the default schema to the Core schema

[Paalon]

According to the YAML spec, a YAML processor should use the Core schema as its default.

From YAML spec. 1.2.2, 10.3. Core Schema:

The Core schema is an extension of the JSON schema, allowing for more human-readable presentation of the same types. This is the recommended default schema that YAML processor should use unless instructed otherwise. It is also strongly recommended that other schemas should be based on it.

Currently (in YAML.jl v0.4.10), we use an original schema, so it will be a breaking change and should keep the functionality as an optional schema and gradually deprecate it.

[Paalon]

First, we should solve #151, then let's get started on this.