chore: Fix key generation with boxed RNG

[0xsimka]

I noticed that key generation was failing when using a boxed RNG.

Updated all instances of:

Keypair::new(&secp, &mut self.rng)

to:

Keypair::new(&secp, &mut *self.rng)

This ensures self.rng (a Box<dyn RngCore>) is correctly passed as &mut dyn RngCore.
The change fixes the mismatch and allows key creation to work as intended.

[mattsse]

this currently works as expected