replace iotools with vroom **WIP**

[jeffeaton]

In read_dhs_flat(), replace iotools::input.file() with vroom::vroom() for reading fixed-width text files.

vroom::vroom is much faster and memory efficient. Makes all parsing faster and improves ability to read very large datasets without exhausting system memory (i.e. India DHS surveys).

Also replaces Map() with for() loop when assigning variable labels to avoid full dataset copy in memory.

Work in progress

This PR works, but two remaining things to do:

• vroom::vroom() is pretty chatty and throws lots of messages / warnings. Probably want to silence some of these.
• Run a systematic download of all datasets to ensure no edge cases or unexpected issues.