Outlaw the "\ddd" notation

[Agent Zhang]

Hi, there~

Perl 5 uses the "\ddd" notation to index characters by octal numbers
(e.g. \187 and \13). Now that Perl 6 has the shiny new \o and \o[]
notations, we probably need to outlaw the legacy stuff explicitly in
S02 since we have the assumption that everything not mentioned in the
Synopses is assumed to stay the same as in Perl 5.

Therefore, in Perl 6, "\187" eq '187' will hold.

BTW, Audrey also thinks it's necessary to add a snippet to S02. :=)

Cheers,
Agent