assorted improvements to Metafont tests

Stop dropping leading zeros after decimal point

An arbitrary MF path must still have C0 continuity

Need lookAhead for last point of segment in order to parse multiple segments

Print minus sign for numbers in the interval (-1,0)

limit Arbitrary values to the range of MetaFont's fixed precision
arithmetic

Add NFData instances to check for (abuse of) partial functions

Add comparative SVG output with control points
Don't take the head of an empty list
Fix sign error in direction of straight segments
code cleanup, no change of meaning
code cleanup
Copy directions from control joins also

CopyDirsR is only called after controlPtDirs, so the correct direction
will be copied.