Added ProcessStream to for running processes (on Linux) and streaming their output.

Added map() function for creating a mapping iterator.
Added mijin::pipe types for map() and join().

source/mijin/io/stream.hpp

@@ -48,10 +48,11 @@ enum class FileOpenMode
     READ_WRITE
 };
 
-enum class StreamError
+enum class [[nodiscard]] StreamError
 {
     SUCCESS,
     IO_ERROR,
+    NOT_SUPPORTED,
     UNKNOWN_ERROR
 };