* fix(extract `timestamp_checked_add` as lib)
* fix(whisper types): remove unused `EmptyTopics`
* fix(time-lib): feature-flag to use time-lib or std
This commit adds conditional compilation checks that falls back to `our time-lib` when
`time_checked_add` is not available in the standard library
Note, `time_checked_add` covers both `checked_add` and `checked_sub`
* fix(grumble): use cfg_attr to define rustc feature
Artem Vorotnikov
Niklas Adolfsson