Switch to C++17

[ochafik]

llama.cpp just adopted it, and that's also what gemma.cpp uses.

Some options:

• Retire nonstd_make_unique backfill
• Destructure pairs
• std::optional for attempted parsing
• std::variant for Value?
• More elegant function / filter definitions (leverage new auto inference)
• (in the process, optionally make the dep to json a bit looser to be able to switch to json.cpp)