Add .save property to XoshiroEngine, Xoroshiro128Plus, PermutedCongruentialEngine, and PhobosRandom wrapper

Makes mir.random PRNGs that satisfy Phobos std.random.isUniformRNG also satisfy std.range.primitives.isForwardRange. Affected PRNGs and templates:

• mir.random.engine.xoshiro: Xoshiro256StarStar, Xoshiro128StarStar_32, Xoroshiro128Plus, XoshiroEngine.
• mir.random.engine.pcg: pcg32, pcg32_oneseq, pcg32_fast, pcg8_once_insecure, pcg16_once_insecure, pcg32_once_insecure, pcg64_once_insecure, pcg8_oneseq_once_insecure, pcg16_oneseq_once_insecure, pcg32_oneseq_once_insecure, pcg64_oneseq_once_insecure; PermutedCongruentialEngine as long as output_previous is true and streamType is not stream_t.unique.
• mir.random.engine.PhobosRandom!Engine as long as the representation of Engine contains no pointers other than function pointers and its opCall() and destructor (if any) are @safe and pure. All PRNGs defined in mir.random meet these conditions.

Prior to this the only mir.random PRNGs that implemented .save were mir.random.engine.splitmix.SplitMix64 and Splittable64.