Add class DTCollation_numeric

We should eventually remove DTCollation from the top level Item class. Only string types really need it. It wastes memory for non-string Items.

As the first step, let's do the following:

• add a new class DTCollation_numeric
• change all:

  collation.set_numeric()

  to:

  collation= DTCollation_numeric();

• remove virtual methods Field::derivation() and Field::repertoire()
• change Field::dtcollation() to be a virtual method:

  virtual DTCollation dtcollation()= 0;

• Reuse DTCollation_numeric() in Field_numeric::dtcollation() and Field_temporal::dtcollation()
• Replace these members in Field_str:

  // TODO-10.2: Reuse DTCollation instead of these three members

  CHARSET_INFO *field_charset;

  enum Derivation field_derivation;

  uint field_repertoire;

  to:

  DTCollation m_collation;

Later we'll remove DTCollation from the top level Item class and replace it to a new Item virtual method:

virtual DTCollation dtcollation();

So numeric and temporal methods will return and instance of DTCollation_numeric(), by value or by reference.