message: Add conversion rules to stratification
diff: %extend context.
  - eval : context -> stratified.kind -> kind_value -> prop.
  - % TODO: eval
  + eval : context -> stratified -> value -> prop.
  - eval : context -> stratified.typ -> typ_value -> prop.
  - % TODO: eval
  -
  - eval : context -> stratified.term -> term_value -> prop.
  - % TODO: eval
  + is_equal : context -> value -> value -> prop.
  %end.
  % ```
  % Conversion
  + is_type Context Elem Elem' :-
  + check_type Context Elem (kind type_type) Elem'.
  + is_type Context Elem Elem' :-
  + check_type Context Elem (kind format_type) Elem'.
  + check_type Context Elem Type Elem' :-
  + synth_type Context Elem Type' Elem',
  + context.is_equal Context Type Type'.
  +
  % Variables
  % Annotated terms
  is_type Context (core.ann Elem Type) Elem' :-
  - is_type Context Type (kind Kind),
  + is_type Context Type Kind,
  context.eval Context Kind Kind',
  - check_type Context Elem (kind Kind') Elem'.
  - is_type Context (core.ann Elem Type) Elem' :-
  - is_type Context Type (typ Type'),
  - context.eval Context Type' Type'',
  - check_type Context Elem (typ Type'') Elem'.
  + check_type Context Elem Type' Elem'.
  % Universes
  is_type Context core.type_type (kind stratified.type_type).
  