util: add BitSet

This adds a bitset module that implements a BitSet<N> class, a variant
of std::bitset with a few additional features that cannot be implemented
in a wrapper without performance loss (specifically, finding first and
last bit set, or iterating over all set bits).

Author: sipa

src/Makefile.am

@@ -292,6 +292,7 @@ BITCOIN_CORE_H = \
   util/batchpriority.h \
   util/bip32.h \
   util/bitdeque.h \
+  util/bitset.h \
   util/bytevectorhash.h \
   util/chaintype.h \
   util/check.h \