Fix Conduit 1.3 deprecation warnings
- The `Conduit`, `Producer` and `Consumer` aliases are deprecated and
`ConduitT` is used directly instead
- `=$=` is deprecated and replaced by `.|`
Add support for logging to tensorboard
Based on @gnezdo's internal version with some differences:
* Uses a pure haskell implementation of EventWriter instead of FFI.
* Special `buildAnd*` functions were dropped in favor of using
`mergeAllSummaries :: Build SummaryTensor` with the normal
`build` function.
