For POSIX, TTYs are never regular files, so if the interpreter knows the
file is regular it doesn't need to do an additional system call to check
if the file is a TTY.

The `open()` Python builtin requires a `stat` call at present in order
to ensure the file being opened isn't a directory. That result includes
the file mode which tells us if it is a regular file. There are a number
of attributes from the stat which are stashed one off currently, move to
stashing the whole object rather than just individual members.

The stat object is reasonably large and currently the
`stat_result.st_size` member cannot be modified from Python, which is
needed by the `_pyio` implementation, so make the whole stat object
optional. In the `_io` implementation this makes handling a stat
failure simpler. At present there is no explicit user call to clear it,
but if one is needed (ex. a program which has a lot of open FileIO
objects and the memory becomes a problem) it would be straightforward to
add. Ideally would be able to automatically clear (the values are
generally used during I/O object initialization and not after. After a
`write` they are no longer useful in current cases).

It is fairly common pattern to scan a directory, look at the `stat`
results (ex. is this file changed), and then open/read the file. In this
PR I didn't update open's API to allow passing in a stat result to use,
but that could be beneficial for some cases (ex. `importlib`).

With this change on my Linux machine reading a small plain text file is
down to 6 system calls.

```python
openat(AT_FDCWD, "read_one.py", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=87, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "from pathlib import Path\n\npath ="..., 88) = 87
read(3, "", 1)                          = 0
close(3)                                = 0
```

…anding buffer