Refactor RNGs to support Windows Phone

This OS has its own crypto API and does not support CryptGenRandom.

Splits System_RNG_Impl into distinct declarations one per implementation
type. Easier to read now that we are up to 4 distinct versions.

Removes the CryptoAPI entropy source, and replaces it with an entropy
source that calls the system RNG. This is nominally a bit less flexible
in that the entropy source allowed polling multiple providers (though
we didn't actually make use of that). Plus side is it works on all
systems.

Currently the dev_random entropy source is still there because we do
actually use it to poll both /dev/random and /dev/urandom, and it
might be useful (on certain systems) to also poll a HW RNG, which
are often assigned their own device node. This could debatably also
be removed in favor of just reading the system RNG.

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