Remove data.table dependency

[inpowell]

This PR removes modulartabler's dependency on data.table in lieu of tidyr pivoting. This comes with a couple of benefits:

1. convert_tabular() now uses reshaping functions from tidyr. This means we do not have to coerce multiple measures to the same type, as happens with melt/dcast.
2. tidyr is compatible with vctrs classes, like in maskr which I plan to implement later.
3. We will gain (a better shot) at compatibility with dbplyr, dtplyr, duckplyr and other lazy-evaluation pipeline packages.

This does result in some losses:

1. There is no conversion shim argument to convert measures before reshaping. This was used to prevent a warning from data.table, so is not as important now.
2. There is no value_override argument, which was used to create a suppression mask table. This will not be required when maskr gets introduced, so is hopefully only a temporary loss.

This also provided an opportunity to add examples and improve documentation.