[RFC] More visibility for YAML::Schema::FailSafe

[straight-shoota]

Crystal's YAML parser supports the failsafe schema, but it's quite hidden in YAML::Schema::FailSafe and not mentioned anywhere else. In contrast, the Core schema is prominently used by YAML.parse.

Now the API docs for YAML should certainly at least mention this alternative schema.

But I think there should also be an easier option for using it. Compared to the short code YAML.parse(yaml), YAML::Schema::FailSafe.parse(yaml) is quite bulky and resides two nested namespaces deeper.

I'd like to introduce an easily accessible API for the failsafe schema directly on YAML for convenience.
Options for this are:

• an additional overload which accepts a parameter: YAML.parse(yaml, schema: :fail_safe)
• a separate method: YAML.parse_fail_safe(yaml)

Failsafe is the smallest sibling of the YAML schemas and a tiny subset of the Core schema. It only supports mapping, list, and string types. This makes it much more sane than its bigger siblings (for example, it doesn't interpret Norway as a Boolean). It's entirely sufficient for many use cases of parsing a dynamic YAML structure (YAML::Any). I think it should often be preferred over the Core schema.