Allow forward-declaration of level_enum

[daverigby]

spdlog::level::level_enum cannot be forward-declared at present, as
the definition does not specify an underlying type.

To allow users to make use of <spdlog/fwd.h> to refer to
level::level_enum without pulling in all of <spdlog/common.h> (which
can be quite costly), specify an underlying type (int) for
level::level_enum, then add a forward-declaration for it to
spdlog/fwd.h.

Note this required explicitly casting level_enum to size_t within ansicolor_sink due to sign-conversion errors:

implicit conversion changes signedness: 'const level::level_enum' to 'std::__1::array::size_type' (aka 'unsigned long') [-Wsign-conversion]

It would appear that an enum with an unspecified underlying type is in
some kind of superposition - it can be treated as both signed and
unsigned - using an underlying type of 'unsigned int' triggers even
more warnings of this kind...

[gabime]

Thanks @daverigby