Simplify SHAMapItem construction:

The existing class offered several constructors which were mostly
unnecessary. This commit eliminates all existing constructors and
introduces a single new one, taking a `Slice`.

The internal buffer is switched from `std::vector` to `Buffer` to
save a minimum of 8 bytes (plus the buffer slack that is inherent
in `std::vector`) per SHAMapItem instance.

src/ripple/protocol/impl/Serializer.cpp

@@ -104,6 +104,14 @@ Serializer::addRaw(Blob const& vector)
     return ret;
 }
 
+int
+Serializer::addRaw(Slice slice)
+{
+    int ret = mData.size();
+    mData.insert(mData.end(), slice.begin(), slice.end());
+    return ret;
+}
+
 int
 Serializer::addRaw(const Serializer& s)
 {