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.

test/sanitizer_suppressions/ubsan

@@ -77,3 +77,4 @@ shift-base:streams.h
 shift-base:FormatHDKeypath
 shift-base:xoroshiro128plusplus.h
 shift-base:RandomMixin<*>::randbits
+shift-base:RandomMixin<*>::randbits<*>