Remove method dispatch from cipher and pk

### Description

https://github.com/ARMmbed/mbed-crypto/pull/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

Start by reviewing PR 185 and the existing md-module changes, then locate the corresponding dynamic method-dispatch paths for cipher and pk. Compare both modules for consistency with that precedent; done means dispatch is removed from both while the existing test suite still passes.

Written by the indexing model from the issue text.