Avoid magic numbers

[AnomalRoil]

These values (48, 32, 32) might change if we were to start using another curve than BLS12-381:

tlock/encoders/base/base.go

Lines 89 to 99 in 78dae76

	kyberPoint, err := readPayloadBytes(rr, 48)
	if err != nil {
	return tlock.CipherInfo{}, fmt.Errorf("failed to read kyber point: %w", err)
	}
	cipherV, err := readPayloadBytes(rr, 32)
	if err != nil {
	return tlock.CipherInfo{}, fmt.Errorf("failed to read cipher v: %w", err)
	}
	cipherW, err := readPayloadBytes(rr, 32)

Not sure what's the best solution:

• have them be const in a constant.go file?
• have a way to determine them from the encrypted file?

WDYT @nikkolasg ? Do we have them somewhere already as part of the current "scheme-name" drand is using? I don't think so.

[nikkolasg]

The length are present in the suite
https://github.com/drand/kyber/blob/master/group.go#L171-L174
So using the suite as in drand will make these generic over the curve's length

[ardan-bkennedy]

I have added constants for everything and included calculating the size of the PEM labels. If you decide to add these constants the code can be easily changed.

[ardan-bkennedy]

There are no more magic numbers. Everything has a constant.