Keep quotation marks when printing strings (#225)

Author: MasonProtter

src/types.jl

@@ -425,6 +425,7 @@ cdrargs(args) = setargs(t, cdr(args))
 print_arg(io, x::Union{Complex, Rational}) = print(io, "(", x, ")")
 print_arg(io, x) = print(io, x)
 print_arg(io, f::typeof(^), x) = print_arg(IOContext(io, :paren=>true), x)
+print_arg(io, s::String) = show(io, s)
 function print_arg(io, f, x)
     f !== (*) && return print_arg(io, x)
     if istree(x) && Base.isbinaryoperator(nameof(operation(x)))