Replace Mersenne Twister with a cryptographic PRNG

[axch]

For example, on @riastradh-probcomp has written:

http://mumble.net/~campbell/python/prng.py (32-byte state w/ 32-byte buffer, lower throughput) http://mumble.net/~campbell/python/weakprng.py (48-byte state w/ 64-byte buffer, higher throughput) http://mumble.net/~campbell/python/chacha.py (crypto core PRF)

Why?

• Mersenne Twister's state is a cumbersome 5 kilobytes, impeding things like dumping it to screen during test runs (e.g., for #139).
• I continue to be uneasy about Mersenne Twister's statistical properties if used in complex parallel patterns, as Venture now does. One source of uneasiness: Matsumoto et al.'s "Common Defects in Initialization of Pseudorandom Number Generators."
• I am similarly uneasy about Mersenne Twister's statistical properties when one uses Mersenne Twister to generate seeds for many (relatively short-lived) Mersenne Twister streams, as Venture is now wont to do in various hard-to-predict circumstances.

Any cryptographic PRNG makes the latter two concerns vanish entirely; and at least ChaCha's full state is small enough to print out on one 80-character line using only non-special printable ASCII characters.

[axch]

366 would make this saner and higher performance, but does not strike me as strictly necessary.

[riastradh-probcomp]

C implementation of weakprng here:

https://github.com/probcomp/crosscat/blob/4f75431b06978c77fc1c8e9c559af0f68101316d/cpp_code/src/weakprng.cpp https://github.com/probcomp/crosscat/blob/4f75431b06978c77fc1c8e9c559af0f68101316d/cpp_code/include/CrossCat/weakprng.h

(Nominally C++98 for build convenience but it is valid C99 too.)