Move ArgMap::init into format.h

[Yelnats321]

Currently, internal::ArgMap<Char>::init is defined inside format.cc and explicitly instantiated for char and wchar_t, even though the function itself is char type agnostic. This leads to issues using a character type other than these two. The easiest solution is to simply move the function definition into format.h. I am unsure on the compile time impact of such a change, although it would seem to be negligible, especially considering that most other functions are not explicitly instantiated (with the exception of format_float for understandable reasons and FixedBuffer::grow).

[vitaut]

Seems reasonable. Could you by any chance submit a PR?

[Yelnats321]

Yeah sure just give me a day or two.