Type inference/checking for builtins

[phipsgabler]

The internal logic doesn't accept Core.Builtin types, for which return_types fails:

julia> map(tuple, NumericParser(Int))
ERROR: ArgumentError: argument is not a generic function
Stacktrace:
 [1] return_types(f::Any, types::Any, interp::Core.Compiler.NativeInterpreter)
   @ Base ./reflection.jl:1238
 [2] return_types
   @ ./reflection.jl:1236 [inlined]
 [3] infer_result_type(::Function, ::Type, ::NumericParser{TextParse.Numeric{Int64}, Int64}, ::String; throw_empty_union::Bool)
   @ CombinedParsers ~/.julia/packages/CombinedParsers/kbHhW/src/transformation.jl:330
 [4] map(::Function, ::NumericParser{TextParse.Numeric{Int64}, Int64}; throw_empty_union::Bool)
   @ CombinedParsers ~/.julia/packages/CombinedParsers/kbHhW/src/transformation.jl:284
 [5] map(::Function, ::NumericParser{TextParse.Numeric{Int64}, Int64})
   @ CombinedParsers ~/.julia/packages/CombinedParsers/kbHhW/src/transformation.jl:284
 [6] top-level scope
   @ REPL[6]:1

Expected result: a parser with return type Tuple{Int}.

(I know that map(x -> tuple(x), NumericParser(Int)) works instead, but that's rather redundant...)