std.file: Replace FileException with OSException

FileException is now an alias of OSException.

Related changes:
- Removed some redundant stat calls
- Improved copy's exception text on Windows
std.file: Replace enforce calls in unit tests with assert
std.file: Add exception specification to slurp
std.file: Replace a few enforce calls with proper alternatives
std.stdio: Replace private wenforce with std.windows.syserror one
std.exception: Improve ErrnoException based on StdioException
std.stdio: Make StdioException a subclass of ErrnoException
std.process: Replace uses of StdioException
std.stdio: Replace uses of StdioException
std.stdio: Replace StdioException with an alias
std.stdio: Remove unused private binaryWrite function
std.stdio: Update documentation
std.process: Replace ProcessException with OSException
std.file: Update documentation and unit tests
std.datetime: Update documentation
std.mmfile: Update documentation
std.process: Replace uses of sysErrorString
std.windows.charset: Replace uses of sysErrorString
std.stdio: XREF exceptions in documentation