Consider replacing PyYAML with strictyaml

[janezd]

Trubar uses a very limited subset of Yaml, so the parser should be as simple as possible, if this is beneficial in any way.

PyYAML's error messages are difficult to understand. I hope strictyaml's are better, because it understands a limited set of YAML.

It should also be safer, e.g., avoids the Norway problem etc.

[janezd]

• pyyaml needs 1-2 seconds to read a 400 KB file https://github.com/biolab/orange-translations/blob/master/orange3/si.yaml
• strictyaml needs ~10 seconds without schema and ~30 seconds with schema.

Furthermore, trubar's schema is recursive. MsgDict = Dict[str, Union["MsgDict", str, bool, None]] can be represented in strictyaml using the following hack,

yaml_schema = MapPattern(Str(), EmptyNone() | NullNone() | Bool() | Str() | Validator())
yaml_schema._value_validator._validator_b = yaml_schema

but we obviously don't want to do this.

Continued in #41.