Uvector print options

[shirok]

Add options to <write-controls> to customize uniform vector output.

• Rafix. Sometimes it's useful to dump uvector in hexadecimals, etc.
• If we use base >= 10, whether we use uppercase A-Z or lowercase a-z.
• Use string-notated bytevector (srfi-207) for u8vectors

This may be done with format directives, but these kind of options tend to get arbitrarily complex and we'll end up another bloated cryptic mini-language. Using <write-controls> may be easier to handle.