Support for OSX

[vaivaswatha]

• Use boost's memory_resource instead of from std for Apple (Apple doesn't support C++17 completely yet).
• Sort maps to make printing them deterministic. This includes a functional change to map_to_list to sort it too (see).
• ~Json::toStyledString()~ prettyPrintJSON(): cross-platform formatted JSON printing.
• Link to stdc++.
• Disable warning -Wreturn-type-c-linkage when returning SafeInt in extern "C" functions.
• Other minor changes.
• CI support.