Change union value type to contain uint8_t types instead of char.

Make the members of the union value type in src/data/value.h be
uint8_t instead of char.  This is more logical since the contents
of values cannot be considered "strings" until they have been
formatted.  The unformatted values are merely arrays of bytes.

This has the added advantage of provoking compiler warnings when
a char * type is being implicitly cast to a uint8_t * or vici-versa.
When such a warning is encountered, is probably means that the
data needs to be re-encoded using recode_string.