branches: 1.1.2; 1.1.4; 1.1.6; 1.1.8; 1.1.10; 1.1.18;
Switch from NIST CTR_DRBG with AES to NIST Hash_DRBG with SHA-256.

Benefits:

- larger seeds -- a 128-bit key alone is not enough for `128-bit security'
- better resistance to timing side channels than AES
- a better-understood security story (https://eprint.iacr.org/2018/349)
- no loss in compliance with US government standards that nobody ever
got fired for choosing, at least in the US-dominated western world
- no dirty endianness tricks
- self-tests

Drawbacks:

- performance hit: throughput is reduced to about 1/3 in naive measurements
=> possible to mitigate by using hardware SHA-256 instructions
=> all you really need is 32 bytes to seed a userland PRNG anyway
=> if we just used ChaCha this would go away...

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