serialization: protect blob serialization from undefined behavior

There is currently no compiler protection when someone tries to
do (for example) `BLOB_SERIALIZER(std::vector<int>)`. You just
get runtime allocation errors. This has already eaten up dev time
before, so this PR adds a static assertion that the type must be
trivially copyable, as defined by the C++ standard. Types can
override this if applicable if they use `BLOB_SERIALIZER_FORCED`.

tests/unit_tests/serialization.cpp

@@ -51,6 +51,11 @@
 using namespace std;
 using namespace crypto;
 
+static_assert(!std::is_trivially_copyable<std::vector<unsigned char>>(),
+  "should fail to compile when applying blob serializer");
+static_assert(!std::is_trivially_copyable<std::string>(),
+  "should fail to compile when applying blob serializer");
+
 struct Struct
 {
   int32_t a;