Loosen File and Stream filename type constraints.

Maintains the same public API (e.g., type constraints on methods remains the same),
but allows FilePaths.jl to extend the public methods to work with `AbstractPath`s.
This seemed preferable to depending directly on FilePathsBase or loosening the type constraints on all methods.

src/types.jl

@@ -26,7 +26,7 @@ DataFormat `fmt`.  For example, `File{fmtpng}(filename)` would indicate a PNG
 file.
 """
 struct File{F<:DataFormat} <: Formatted{F}
-    filename::String
+    filename
 end
 File(fmt::Type{DataFormat{sym}}, filename) where {sym} = File{fmt}(filename)
 
@@ -53,7 +53,7 @@ be used to improve error messages, etc.
 """
 struct Stream{F <: DataFormat, IOtype <: IO} <: Formatted{F}
     io::IOtype
-    filename::Union{String, Nothing}
+    filename
 end
 
 Stream(::Type{F}, io::IO) where {F<:DataFormat} = Stream{F,typeof(io)}(io, nothing)