message: Make source and target explicit in variable names
diff: % ```
  % Conversion
  - is_type Context SourceElem Elem:-
  - synth_type Context SourceElem type_type Elem.
  - is_type Context SourceElem Elem :-
  - synth_type Context SourceElem format_type Elem.
  - check_type Context SourceElem Type Elem :-
  - synth_type Context SourceElem Type' Elem,
  - context.is_equal Context Type Type'.
  + is_type Context SourceElem TargetElem:-
  + synth_type Context SourceElem type_type TargetElem.
  + is_type Context SourceElem TargetElem :-
  + synth_type Context SourceElem format_type TargetElem.
  + check_type Context SourceElem TargetType TargetElem :-
  + synth_type Context SourceElem TargetType' TargetElem,
  + context.is_equal Context TargetType TargetType'.
  % Variables
  - synth_type Context (surface.name "Type") Type core.type_type.
  - synth_type Context (surface.name "Format") Type core.format_type.
  - synth_type Context (surface.name Name) Type (core.local Level) :-
  - context.lookup_type Context Name Level Type.
  + synth_type Context (surface.name "Type") TargetType core.type_type.
  + synth_type Context (surface.name "Format") TargetType core.format_type.
  + synth_type Context (surface.name Name) TargetType (core.local Level) :-
  + context.lookup_type Context Name Level TargetType.
  % Annotated terms
  - synth_type Context (surface.ann SourceElem SourceType) Type' (core.ann Elem Type) :-
  - is_type Context SourceType Type,
  - context.eval Context Type Type',
  - check_type Context SourceElem Type' Elem.
  + synth_type Context (surface.ann SourceElem SourceType) TargetType'
  + (core.ann TargetElem TargetType)
  + :-
  + is_type Context SourceType TargetType,
  + context.eval Context TargetType TargetType',
  + check_type Context SourceElem TargetType' TargetElem.
  % Universes
  % Functions
  