Add `src/cpp-common/std-int.hpp`

Add `src/cpp-common/std-int.hpp`

This new header contains the `StdIntT` trait structure to get the
standard integer type from a length (bits) and a signedness.

For example, `bt2_common::StdIntT<32, true>::Type` is `std::int32_t`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I69fc9ac7e01c2a82e4ad13afa553ef8e8720bd57
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8086
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10839
Tested-by: jenkins <jenkins@lttng.org>