Various GValue functions use G_IS_VALUE and G_TYPE_IS_VALUE to check
whether the provided GValue or GType is valid. The problem is two-fold:
* Those checks are expensive (sometimes represent more than 50% of the
  total cost of the function!)
* Later calls to g_type_value_table_peek will validate whether the
  GValue (and type) is valid for usage

This commit replaces most calls to G_IS_VALUE and G_TYPE_IS_VALUE
By simpler checks and delegate the full validation to later function
calls

Speeds up these various functions between 20% and .. 300%