span: Make Span template deduction guides work in SFINAE context

Also add test to make sure this doesn't get broken in the future.

This was breaking vector<bool> serialization in multiprocess code because
template current deduction guides would make it appear like vector<bool> could
be converted to a span, but then the actual conversion to span would fail.

src/Makefile.test.include

@@ -144,6 +144,7 @@ BITCOIN_TESTS =\
   test/sigopcount_tests.cpp \
   test/skiplist_tests.cpp \
   test/sock_tests.cpp \
+  test/span_tests.cpp \
   test/streams_tests.cpp \
   test/sync_tests.cpp \
   test/system_tests.cpp \