Add documentation, expand error handling, implement file and buffer io

include/bh/internal/file_posix.h

@@ -0,0 +1,9 @@
+#include <unistd.h>
+
+struct bh_file_s
+{
+    bh_io_t base;
+    char *path;
+    int mode;
+    int handle;
+};