Unify definition of common escapes across strings and clobs

[mijamo911]

For strings:

common_escape ::= '' ( 'a' | 'b' | 't' | 'n' | 'f' | 'r' | 'v' | '?' | '0' | '' | '/' | 'U' | 'u' | 'x' )

For clobs:

common_escape ::= '' ( 'a' | 'b' | 't' | 'n' | 'f' | 'r' | 'v' | '?' | '0' | '' | '/' | 'x' )

Because the names are the same and they're defined just a few paragraphs apart, it isn't immediately obvious that the escapes differ. Also "common" implies that they are, well, common. I think it would make more sense to make the clob definition common_escape and move \u and \U into a separate unicode_escape production.