SDK v15.3.0

New library features

• All of the below only support the c64. Picked this arbitrarily; other backends will come later.
• fopen
  • There's not actually much you can do with open files, and if you try to use most stdio functions and fopen in the same project, the link will fail.
• fclose (with automatic fclose on exit)
• rename
• tmpfile
• tmpnam