optimise svg and dxf exports

Here is a basic script that produce a rounded box:

```py
import cadquery as cq
from cadquery import exporters

result = ( cq
.Workplane('XY')
.box(50, 50, 1)
.edges('|Z')
.fillet(10)
)

exporters.export(result, 'test.svg', opt={
'width': 300,
'height': 300,
'marginLeft': 10,
'marginTop': 10,
'showAxes': False,
'projectionDir': (0, 0, 1),
'strokeWidth': 0.1,
'strokeColor': (0, 0, 0),
'hiddenColor': (0, 0, 255),
'showHidden': False
})
```

Only one path is necessary for 2d export, but if we look at the source, we can see that many paths are created:

```xml










```

This can be very annoying when using a cnc or laser cutter, because the tool will make a lot of additional useless movements and wille take much more time to cut the piece.

Here is a simulation of what a machine will do on the exported svg (svg moves are replaced with lines):