'const' annotation not supported

[meowsbits]

The JSON Schema libraries which
etclabscore/go-openrpc-reflect depends on DO NOT
support the 'const' annotation, although
it is specd here: https://tools.ietf.org/html/draft-handrews-json-schema-validation-02#section-6.1.3

• https://github.com/alecthomas/jsonschema/blob/master/reflect.go#L34-L77
• https://github.com/go-openapi/spec/blob/master/schema.go#L160-L196

The workaround for this is to use a single item
in an 'enum' instead, which the spec explains to
be functionally equivalent.