* Remove conversion compiler warning

When compiling with g++8, I get the following two errors:
include/fmt/format-inl.h:400:29: error: conversion from ‘int’ to ‘char’ may change value [-Werror=conversion]
       buffer[size++] = zero + static_cast<char>(digit);
                        ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
include/fmt/format-inl.h:416:28: error: conversion from ‘int’ to ‘char’ may change value [-Werror=conversion]
       buffer[size++] = '0' + digit;
                        ~~~~^~~~~~~

With this change, the errors are gone.

This reverts commit 2a952dd0.

Remove the dependency on std::locale to reduce executable sizes

For embedded target the usage of locale bring a huge amount of code to the executable, just to get hold of the thousands separator. Using a static defined thousands separator makes libfmt embedded target friendly.

Since the commit 691a7a91 compile-time string checking was accidently not possible anymore.
With this change, it should be possible again.