Make dump print const before const fields

This provides more information to the user when dumping types, and also makes the
output of dump slightly more similar to the type definition syntax.

Before:
```
julia> dump(BitSet)
BitSet <: AbstractSet{Int64}
  bits::Vector{UInt64}
  offset::Int64
```

After:
```
julia> dump(BitSet)
BitSet <: AbstractSet{Int64}
  const bits::Vector{UInt64}
  offset::Int64
```

Author: jakobnissen

base/show.jl

@@ -3007,7 +3007,9 @@ function dump(io::IOContext, x::DataType, n::Int, indent)
         fieldtypes = datatype_fieldtypes(x)
         for idx in 1:length(fields)
             println(io)
-            print(io, indent, "  ", fields[idx])
+            print(io, indent, "  ")
+            isconst(x, idx) && print(io, "const ")
+            print(io, fields[idx])
             if isassigned(fieldtypes, idx)
                 print(io, "::")
                 print(tvar_io, fieldtypes[idx])