Mark deprecated functionality with attributes and remove usage of such features

[eliaskosunen]

This PR marks features commented as deprecated with attributes; [[deprecated]] if compiled with C++14 and __attribute__((deprecated)) or __declspec(deprecated) if not.

I was able to replace most of the cases where deprecated functionality was used in the library, but one case lingers: https://github.com/eliaskosunen/fmt/blob/599632ee226d74d6c34d89290ebabd7320c90afe/include/fmt/format.h#L3061

arg_map::init takes const basic_format_args<Context>& as its sole argument, which can only be extracted from context_base with the deprecated member function args().

[eliaskosunen]

Travis builds are erroring because of -Werror and the usage of deprecated context_base::args().

[vitaut]

This is great, thanks!

arg_map::init takes const basic_format_args& as its sole argument, which can only be extracted from context_base with the deprecated member function args().

I suggest making args protected and adding a using declaration in basic_format_context for backward compatibility. Not sure if using declaration can be deprecated.

[eliaskosunen]

[[deprecated]] cannot be used with using-declarations, to that option is off the table. I circumvented the issue by simply commenting the FMT_DEPRECATED out and adding a comment explaining it. I couldn't come up with any alternative ways of fixing the issue apart from some messy ones requiring way too much work. I think this should be fixed at some later time.

There was also some usage of the deprecated format_decimal overload in format-test.cc. For that, I simply disabled the warning with pragmas. I don't know that part of the library all that well, so I'm not really one to fix it.

[vitaut]

Merged, thanks!