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.test.include

@@ -48,6 +48,7 @@ BITCOIN_TESTS =\
   test/base32_tests.cpp \
   test/base58_tests.cpp \
   test/base64_tests.cpp \
+  test/bech32_tests.cpp \
   test/bip32_tests.cpp \
   test/bloom_tests.cpp \
   test/checkblock_tests.cpp \