Adding field names to DecaExpr types

[jpfairbanks]

The current defn of the ADTs for decapodes language uses anonymous fields, which is fine when only Tyler had to write the structural recursion and it was an implementation detail. Now that we will be exporting the DecaExpr types as a core part of the interface, we should add field names

@data Model begin
  ACSetSpec
  Decapodes.DecaExpr
end

@data Term begin
  Var(name::Symbol)
  Lit(name::Symbol)
  Judge(v::Var, dim::Symbol, space::Symbol) # Symbol 1: Form0 Symbol 2: X
  AppCirc1(fs::Vector{Symbol}, arg::Term)
  App1(f::Symbol, x::Term)
  App2(f::Symbol, x::Term, y::Term)
  Plus(args::Vector{Term})
  Mult(args::Vector{Term})
  Tan(x::Term)
end

@data Equation begin
  Eq(x::Term, y::Term)
end

# A struct to store a complete Decapode
@as_record struct DecaExpr
  judgements::Vector{Judge}
  equations::Vector{Equation}
end