Replace istream_input by string_input

This is to be used in unit tests
istream_input may be used in the future to allow interactive use of the code...

src/language/ASTBuilder.cpp

@@ -268,4 +268,4 @@ buildAST(InputT& input)
 }
 
 template std::unique_ptr<language::Node> buildAST(read_input<>& input);
-template std::unique_ptr<language::Node> buildAST(istream_input<>& input);
+template std::unique_ptr<language::Node> buildAST(string_input<>& input);

src/language/ASTBuilder.hpp

@@ -8,7 +8,4 @@
 template <typename InputT>
 std::unique_ptr<language::Node> buildAST(InputT& input);
 
-template <typename InputT>
-std::unique_ptr<language::Node> buildAST(InputT& input);
-
 #endif   // AST_BUILDER_HPP