This commit moves some of the TSM mapper logic that had leaked into the
TSM->Parquer converter back into the mapper. The refactor allows us to
make some previously public APIs private, whilst still providing a
reasonably flexible API.
This commit reduces copying of block data by replacing an inefficient
`remove` call on vectors by with an index tracking approach, leving the
original vectors in place.
It further refactors some of the mapping code DRYing things up.
It improves performance of the `map_field_columns` function by 48%.
```
time: [137.11 us 137.50 us 137.92 us]
change: [-49.095% -48.558% -48.033%] (p = 0.00 < 0.05)
Performance has improved.
```
Andrew Lamb
Edd Robinson