refactor: Store Text instead of String inside assertion

On `sydtest-test`, the difference is: ``` - 657 MiB total memory in use + 358 MiB total memory in use ``` This is actually surprising (e.g. that the numbers are so good) because it should only save space on failing test. I'm also unable to replicate similar numbers on a large codebase (e.g. this commit reduced my memory usage from 2.750 GiB during test to 2.720. Mainly the changes are that in all Assertion, we store `Text` instead of `String`. Note that final benchmark should show a bit more allocations (e.g. during conversion from `ppShow` `String` to `Text`), but then it should show less copy and less total memory usage.