4.31.0: IO changes

This release:

• adds TCP\connect_nd_async(), TCP\connect_async(), TCP\Server and equivalents in Unix\
• refactors file locking:
  • removed the _NON_BLOCKING members of the LockType enum
  • splits lock() into lock() (blocking) and tryLockx() (non-blocking)
  • tryLockx() will throw an AlreadyLockedException if flock() fails with EWOULDBLOCK
• refactors all other IO\ and File\ exceptions:
  • adds OS\Exception, containing an ErrorCode; ErrorCode contains constants used in C errno or h_error constants
  • removes per-operation exceptions except for locking, e.g. File\WriteException
  • adds convenience subclasses of OS\Exception for common cases, such as AlreadyExistsException
• adds IO\SeekableHandle, extended by File\Handle
• adds Debug\dump_async() as a replacement for \var_dump(): if IO\request_output() is used, STDOUT is made non-blocking, so can truncate
• adds optional timeouts to readAsync(), readLineAsync(), and writeAsync()
• contains several bugfixes