Add support for inplace_vector

[20162026]

#1676

[20162026]

It works, but I'm not sure if it is worthy to be merged becouse:

1. only json read is implemented ( beve dose not use emplace_back and would require more modifications, didn't check others)
2. serializing into inplace_vector<char, N> without exceptions is tricky becouse of multiple resize calls
3. beman::inplace_vector is not fully implemented (e.g. <=> is broken, no proper constexpr handling, init list only works with exceptions enabled, build fails with clang fnoexcept)

[stephenberry]

Thanks for making this merge request! I do like the idea of inplace_vector support and this gives a starting point for experimenting with that support. I'll look into them when I get some time. I wouldn't mind merging in a solution that is 90% there and improving it over time.