Breaking: Remove file::rdbuf

[bugdea1er]

file (which extends std::iostream) still provides rdbuf() -> std::streambuf which can be used for lower level I/O

std::filebuf (and tmp::filebuf) only provides open, is_open and close methods:

• open - we don't want the user to be able to reopen another file from the temporary file
• close - we don't want file to exist in a closed state
• is_open - will only return false for relocated file objects