Fix conversions for 32bit size_t

[RichLogan]

When compiling for 32 bit, we have some warnings about unsafe casts, primarily because we use gs::VarUint unwrapping to std::uint64_t as inputs for std::size_ts. This ensures deserialize/decodes on 32 bit platforms will safely bail if it receives objects with lengths > than it's std::size_t::max

[GhostofCookie]

Seeing a lot of copy paste but in general I see no errors with this (assuming limiting to 32bit is what is desired).