Import Bech32 C++ reference code & tests

This includes a reformatted version of the Bech32 reference code
(see https://github.com/sipa/bech32/tree/master/ref/c%2B%2B), with
extra documentation.

src/Makefile.am

@@ -124,6 +124,7 @@ BITCOIN_CORE_H = \
   asyncrpcoperation.h \
   asyncrpcqueue.h \
   base58.h \
+  bech32.h \
   bloom.h \
   chain.h \
   chainparams.h \
@@ -358,6 +359,7 @@ libbitcoin_common_a_SOURCES = \
   amount.cpp \
   arith_uint256.cpp \
   base58.cpp \
+  bech32.cpp \
   chainparams.cpp \
   coins.cpp \
   compressor.cpp \