Add %rh option to print buffer as hex without \x

[lenticularis39]

The new option serves as a more readable variant of %rx. In addition to omitting %rx, it puts spaces between the bytes.

Closes #2305

Example:

# bpftrace -e 'BEGIN { printf("%rh", buf("Hello world", 11)); }'
Attaching 1 probe...
48 65 6c 6c 6f 20 77 6f 72 6c 64

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc and if needed in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[viktormalik]

Looks useful to me, thanks!