Role id calculation for constants in Timelock

[andrew-fleming]

a pseudo follow-up to #249

The decision was made in the above issue to exercise agnosticism regarding role id calculation. Timelock, however, utilizes explicit constants for roles i.e.

const PROPOSER_ROLE = 0x584d52d759b8167ea85b5b15e229930249c790924513d0eae539b0415b40ce6

Opinions on the best approach in Cairo? Consider available tooling and ease of application. For now, the drafted implementation uses the first 251 bits.