Add toyaml, fromyaml context methods

[clrcrl]

Describe the feature

Make toyaml and fromyaml methods.
toyaml should have an optional argument, sort_keys=False.
Similar to tojson, fromjson methods.
I'd use this when using a macro to generate schema.yml files.

Describe alternatives you've considered

Here is how I currently do it:

{% set sources_yaml=[] %}

{% do sources_yaml.append('version: 2') %}
{% do sources_yaml.append('') %}
{% do sources_yaml.append('sources:') %}
{% do sources_yaml.append('  - name: ' ~ schema_name | lower) %}
...

[beckjake]

Fixed in #2036