chore(core): replace std::codecvt in module u16

[SabineSIL]

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Describe the solution you'd like

std::codecvt will be deprecated by C++17. We still use std::codecvt in the module u16 at 3 locations:

• /developer/src/kmcmplib/src/
• mcompile-linux
• mcompile-mac

std::codecvt needs to be replaced there. Tests would also be good

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