dirext: Add APIs to read whole contents optionally

[cgwalters]

This came up in a bootc PR that would have been cleaner with this.

[travier]

Lint error is minor:

error: the borrowed expression implements the required traits
   --> tests/it/main.rs:190:29
    |
190 |     td.write("invalid.bin", &invalid_utf8)?;
    |                             ^^^^^^^^^^^^^ help: change this to: `invalid_utf8`