In order to use bare mp_log contexts, I find myself creating dummy
structs (with a single "log" field) to use the MP_ERR() etc. macros,
which hardcode the idiom that a context struct has a log field. On the
other hand, just using mp_msg_log() is too much typing (and I want to
rename it to mp_msg() when the transition is done), so it seems nice to
have message printing macros that use mp_log directly.