feat: Add a parse(FILE *) interface

The fstream classes are notorious for their non-existent error handling.

This adds a C-style fILE * IO (fopen(), etc.) alternative interface, so
that if a user needs reliable error handling, they can use that, albeit
more inconvenient, but more robust approach.

toml/exception.hpp

@@ -10,6 +10,18 @@
 namespace toml
 {
 
+struct file_io_error : public std::runtime_error
+{
+  public:
+    file_io_error(int errnum, const std::string& msg, const std::string& fname)
+        : std::runtime_error(msg + " \"" + fname + "\": " + std::strerror(errnum)),
+          errno_(errnum)
+    {}
+    int get_errno() {return errno_;}
+  private:
+    int errno_;
+};
+
 struct exception : public std::exception
 {
   public: