add [print-]hexdump

`hexdump` and `print-hexdump` print binary data in hexadecimal form
```
[OFFSET ]B1 B2 B3 ... S1S2S3 ...
...
```
slightly similar to the `xxd` UNIX tool.

* `hexdump` is like `write-string` in that it accepts `:stream`, `:start`, `:end`, etc. as keyword arguments.
* `print-hexdump` is designed for use in `~/` format directives.

Basic unit tests are included.

I hope these functions can be considered to be within the scope of the nibbles library. However, I can understand if they are not.