Isolate metadata serialization from device

Previously, the `mgard_x::Metadata` structure had inline code that used
protobuf to serialize its values into a buffer. This was problematic
because the latest version of protobuf does not compile with nvcc.
The issue is a feature of C++17 does not support (and it is unclear if
it will ever be supported).

Get around this problem by moving (the majority of) the serialization
code into the MGARD library and compile it with the host compiler. To
make this work, the code now has to serialize into a host buffer and
then copy into a device buffer. However, the previous implementation
simply copied things one at a time into the device buffer, so this
should be no less performant.

Author: kmorel