[core] change Illegal escape sequence policy
[core] change Illegal escape sequence policy

Previously: Silently ignore illegal escapes even for a-zA-Z
     and change the string from "foo\o" to "fooo"
Now: Throw "Illegal escape sequence \o in foo\o"
See GH #1103

FROGGS/parrot,FROGGS/parrot,parrot/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,FROGGS/parrot,youprofit/parrot,tkob/parrot,FROGGS/parrot,FROGGS/parrot,tkob/parrot,parrot/parrot,youprofit/parrot,parrot/parrot,tkob/parrot,FROGGS/parrot,parrot/parrot,tkob/parrot,youprofit/parrot,tkob/parrot,tkob/parrot,tkob/parrot,FROGGS/parrot,parrot/parrot,FROGGS/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot