- `v`: adds a tag "version" based on the expression, e.g. `v(VERSION)`
-> `t("version", VERSION)`
- `utc`: like `tm`, but converts a `DateTime<Utc>` to `i64`
automatically

this was to test cost in terms of compilation speed. doesn't seem like a
big deal but leaving it in case it is later.

I copied several Window structs from windows crate to avoid a cyclic
dependency. The rationale was, latency mod is on its way out anyway.

- kills MeasurementWindow/WTen
- all warnings silenced
- cruft removed