Remove method dispatch from cipher and pk

[gilles-peskine-arm]

Description

#185 removes dynamic method dispatch from the md module. This reduces the code size and complexity, at the expense of removing the undocumented ability to define custom md algorithms.

We should do the same for cipher and pk, both to get the same benefits and for consistency.

Issue request type

[ ] Question
[x] Enhancement
[ ] Bug

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/IOTCRYPT-891

[adbridge]

Apologies, this was closed in error by the automated bot - re-opening.