The `highlight' module seems to highlight every digit in
YAML (and possibly other) source files. This causes problems
in linkify_2 which replaces the placeholders with proper URIs.
I suspect `-' and other punctuation characters will cause
similar problems, so we must stick to [A-Za-z].
Thus transliterate 0-9 to A-J in the hex key to ensure highlight
doesn't see digit characters, and rename the prefix to be
project-name independent.