Fix the documentation by adding referenced-but-not-included functions and
some other small fixes.

The only warnings left are:

```
python-rsa/rsa/key.py:docstring of rsa.key.AbstractKey.load_pkcs1:: WARNING: py:class reference target not found: rsa.key.T
python-rsa/rsa/key.py:docstring of rsa.key.AbstractKey.load_pkcs1:: WARNING: py:class reference target not found: rsa.key.T
```

These are due to Sphynx not really understanding `typing` type references.
Not sure how to fix those.

There were very few functions in there, and none of them were actually used
by the RSA library (just by the test code).

`PublicKey` and `PrivateKey` both define the `n` and `e` slots, which
are already present in their base class. This reduces the benefits of
having slots.

```shell
$ slotscheck -m rsa -v
ERROR: 'rsa.key:PrivateKey' defines overlapping slots.
       - e (rsa.key:AbstractKey)
       - n (rsa.key:AbstractKey)
ERROR: 'rsa.key:PublicKey' defines overlapping slots.
       - e (rsa.key:AbstractKey)
       - n (rsa.key:AbstractKey)
```

The Python docs say:

> If a class defines a slot also defined in a base class, the instance
> variable defined by the base class slot is inaccessible (except by
> retrieving its descriptor directly from the base class). This renders
> the meaning of the program undefined.

Use the Chinese Remainder Theorem when decrypting with private key, as that
makes the decryption 2-4x faster.

This fixes #163.