Add overloads to JSON::Value to allow assignment of size_t types (#214)

[charles-dyfis-net]

Addresses build-time issue on MacOS. Not tested on other platforms.

[Thulinma]

On Linux, this change breaks the build: size_t and uint64_t are aliases on 64-bit Linux configurations, and it will complain about a duplicate declaration.

The way we've fixed this so far (there were many more cases of this problem in the past) is to typecast to uint64_t whenever we assign a size_t to a JSON value. Been meaning to go through all the cases we missed, but MacOS is not exactly a priority at the moment since practically none of our customers use it.

It would probably be better to add a precompiler check around a size_t version of these functions, to only add them in if size_t is not already an alias of an existing declaration. 🤔