Silence output and make building multiple examples work properly
Verify that messages are between 86 and 255 characters long

Without any padding at all, messages that are shorter than this won't be
greater than `n` when exponentiated, so the same message will always
"encrypt" to the same value regardless of the value of the key.

Messages longer than 255 characters will usually be greater than 2*`n` when
exponentiated, and will decrypt incorrectly.

All of this is because my implementation is (consciously) terrible at
the moment.
Change the API to return (pubkey,privkey) instead of RSA

Also, change message length validation to only verify message is not too
long. Real RSA implementations use padding to solve the too-short
problem, and ours is just insecure until we do that anyway, so there's
no point doing a bespoke poor attempt at it.

Also, make the public exponent configurable. Still defaults to 3.
Make key size variable, defaults to 1024
Make encrypt/decrypt biguint public