random: add RandomMixin::randbits with compile-known bits

In many cases, it is known at compile time how many bits are requested from
randbits. Provide a variant of randbits that accepts this number as a template,
to make sure the compiler can make use of this knowledge. This is used immediately
in rand32() and randbool(), and a few further call sites.

src/random.cpp

@@ -741,6 +741,6 @@ void RandomInit()
 
 std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval)
 {
-    double unscaled = -std::log1p(GetRand(uint64_t{1} << 48) * -0.0000000000000035527136788 /* -1/2^48 */);
+    double unscaled = -std::log1p(FastRandomContext().randbits<48>() * -0.0000000000000035527136788 /* -1/2^48 */);
     return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);
 }