random: refactor: move rand* utilities to RandomMixin

Rather than make all the useful types of randomness be exclusive to
FastRandomContext, move it to a separate RandomMixin class where it can be reused by
other RNGs.

A Curiously Recurring Template Pattern (CRTP) is used for this, to provide the ability
for individual RNG classes to override one or more randomness functions, without
needing the runtime-cost of virtual classes.

Specifically, RNGs are expected to only provide fillrand and rand64, while all others
are derived from those:
- randbits
- randrange
- randbytes
- rand32
- rand256
- randbool
- rand_uniform_delay
- rand_uniform_duration
- min(), max(), operator()(), to comply with C++ URBG concept.

src/random.cpp

@@ -665,7 +665,7 @@ void FastRandomContext::fillrand(Span<std::byte> output) noexcept
     rng.Keystream(output);
 }
 
-FastRandomContext::FastRandomContext(const uint256& seed) noexcept : requires_seed(false), rng(MakeByteSpan(seed)), bitbuf_size(0) {}
+FastRandomContext::FastRandomContext(const uint256& seed) noexcept : requires_seed(false), rng(MakeByteSpan(seed)) {}
 
 bool Random_SanityCheck()
 {
@@ -715,7 +715,7 @@ bool Random_SanityCheck()
 
 static constexpr std::array<std::byte, ChaCha20::KEYLEN> ZERO_KEY{};
 
-FastRandomContext::FastRandomContext(bool fDeterministic) noexcept : requires_seed(!fDeterministic), rng(ZERO_KEY), bitbuf_size(0)
+FastRandomContext::FastRandomContext(bool fDeterministic) noexcept : requires_seed(!fDeterministic), rng(ZERO_KEY)
 {
     // Note that despite always initializing with ZERO_KEY, requires_seed is set to true if not
     // fDeterministic. That means the rng will be reinitialized with a secure random key upon first
@@ -726,10 +726,8 @@ FastRandomContext& FastRandomContext::operator=(FastRandomContext&& from) noexce
 {
     requires_seed = from.requires_seed;
     rng = from.rng;
-    bitbuf = from.bitbuf;
-    bitbuf_size = from.bitbuf_size;
     from.requires_seed = true;
-    from.bitbuf_size = 0;
+    static_cast<RandomMixin<FastRandomContext>&>(*this) = std::move(from);
     return *this;
 }