WL#8606: Mutator ST_X and ST_Y

Add mutator functions ST_X and ST_Y that change the X and Y values of
a geometric point, according to SQL/MM (ISO/IEC 13249-3:2011)
Sect. 6.1.3 and 6.1.4.

MySQL already has ST_X and ST_Y observer functions. These are modified
to raise ER_UNEXPECTED_GEOMETRY_TYPE instead of NULL if the geometry
argument is a valid geometry, but not a point.

Unless otherwise specified, the terms "ST_X" and "ST_Y" in this
section refers to the two-parameter mutator functions, not the
observer functions.

Functional requirements:

F-1: ST_X and ST_Y MUST return NULL if either argument is NULL.

F-2: ST_X and ST_Y MUST NOT return NULL if neither argument is NULL.

F-3: If the geometry argument is not a valid geometry, ST_X/ST_Y MUST
     raise ER_GIS_INVALID_DATA during function evaluation.

F-4: If the new coordinate value argument is a NaN or +/- Inf value,
     ST_X/ST_Y MUST raise ER_DATA_OUT_OF_RANGE.

F-5: If the geometry argument is a valid point and the new coordinate
     value is a valid coordinate value, ST_X/ST_Y MUST return a point.

F-6: If the geometry argument is a valid geometry, but not a point,
     both observer and mutator variants of ST_X/ST_Y MUST raise
     ER_UNEXPECTED_GEOMETRY_TYPE.

Non-functional requirements:

NF-1: The functions MUST be non-nullable if all arguments are
      non-nullable.

I-1: No new files.

I-2: Interface SQL02 is extended with new two-parameter forms of ST_X and ST_Y:
     <geometry> = ST_X(<geometry>, <double>)
     <geometry> = ST_Y(<geometry>, <double>)

I-3: No new commands.

I-4: No new tools.

I-5: Interface SQL02 is changed so that ST_X(<geometry>) and
     ST_Y(<geometry>) will raise ER_UNEXPECTED_GEOMETRY_TYPE instead
     of returning NULL if <geometry> is geometry of another type than
     point.

I-6: No new errors.

I-7: No new warnings.

Create new classes Item_func_set_x and Item_func_set_y, subclasses of
Item_geometry_func, that set the X and Y coordinates,
respectively.

Verify that the geometry argument is a valid point and return
ER_GIS_INVALID_DATA if it isn't. Verify that the new coordinate value
is not a NaN or Inf value (either sign). Return ER_DATA_OUT_OF_RANGE
if it is.

Rename Item_func_x and Item_func_y to Item_func_get_x and
Item_func_get_y, respectively. Modify Create_func_x and Create_func_y
to accept both 1 and 2 argument forms of ST_X/ST_Y and create the
corresponding item.