Add ORBIT_LOG_VAR utility macro

[pierricgimmig]

int my_int = 3;
ORBIT_LOG_VAR(my_int);

my_int = 3

This is a quality of life macro for development. We need to specify our own "to_string" as std::to_string does not handle string types.

[danielfenner]

lgtm!

[florian-kuebler]

Thanks!

[pierricgimmig]

Merging despite conan issue in the Windows CI which is unrelated to this change:

Error while finding module specification for 'conans.conan' (ModuleNotFoundError: No module named 'conans')