> fs: consistently use fsbridge for {i,o}fstream
> fs: add missing <cassert> include
>
> This is needed to prevent compilation failures once boost is removed,
> however is still correct to include now, and reduces the diff in #20744.
>
> <string> is extracted from the defines because it is used for Windows
> and non-Windows code, i.e get_filesystem_error_message().
> fs: consistently use fsbridge for fopen()
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/24167 | core#24167]]