Add ST_MakeLine (deprecates ST_PointsToLine)

Here we introduce `ST_MakeLine`, which will replace the old GDMS function
`ST_PointsToLine`.  It can accept any number of POINTs or MULTIPOINTs and can be
used in scalar or aggregate (in conjunction with `ST_Accum`) form. See examples
below.

See #41.

## Examples
### Scalar forms
#### POINT to POINT(S)
```mysql
SELECT ST_MakeLine('POINT(1 2 3)'::Geometry, 'POINT(4 5 6)'::Geometry);
```
Answer:    `LINESTRING(1 2 3, 4 5 6)`
```mysql
SELECT ST_MakeLine('POINT(1 2)'::Geometry, 'POINT(4 5)'::Geometry);
```
Answer:    `LINESTRING(1 2, 4 5)`
```mysql
SELECT ST_MakeLine('POINT(1 2)'::Geometry, 'POINT(4 5)'::Geometry, 'POINT(7 8)'::Geometry);
```
Answer:    `LINESTRING(1 2, 4 5, 7 8)`
#### POINT to MULTIPOINT
```mysql
SELECT ST_MakeLine('POINT(1 2)'::Geometry, 'MULTIPOINT(4 5, 12 9)'::Geometry);
```
Answer:    `LINESTRING(1 2, 4 5, 12 9)`
#### MULTIPOINT to MULTIPOINT
```mysql
SELECT ST_MakeLine('MULTIPOINT(1 2, 17 6)'::Geometry, 'MULTIPOINT(4 5, 7 9, 18 -1)'::Geometry);
```
Answer:    `LINESTRING(1 2, 17 6, 4 5, 7 9, 18 -1)`
#### A single MULTIPOINT
```mysql
SELECT ST_MakeLine('MULTIPOINT(1 2, 3 4)'::Geometry);
```
Answer:    `LINESTRING(1 2, 3 4)`

### Aggregate forms
#### POINTs only
```mysql
SELECT ST_MakeLine(ST_Accum(point)) FROM input_table;
```
|point Point|
|-----|
|POINT(1 2)|
|POINT(3 4)|
|POINT(5 6)|
|POINT(7 8)|
|POINT(9 10)|
Answer:    `LINESTRING(1 2, 3 4, 5 6, 7 8, 9 10)`
#### POINTs and MULTIPOINTs
```mysql
SELECT ST_MakeLine(ST_Accum(point)) FROM input_table;
```
|point Geometry|
|-----|
|POINT(5 5)|
|MULTIPOINT(1 2, 7 9, 18 -4)|
|POINT(3 4)|
|POINT(99 3)|
Answer:    `LINESTRING(5 5, 1 2, 7 9, 18 -4, 3 4, 99 3)`
#### MULTIPOINTs only
```mysql
SELECT ST_MakeLine(ST_Accum(multi_point)) FROM input_table;
```
|multi_point MultiPoint|
|-----|
|MULTIPOINT(5 5, 1 2, 3 4, 99 3)|
|MULTIPOINT(-5 12, 11 22, 34 41, 65 124)|
|MULTIPOINT(1 12, 5 -21, 9 41, 32 124)|
Answer:    `LINESTRING(5 5, 1 2, 3 4, 99 3, -5 12, 11 22, 34 41, 65 124, 1 12, 5 -21, 9 41, 32 124)`