fill in most of the remaining MetaFont functions

Add functions to convert MetafontSegments to Diagrams Segments
add parser from MetaFont syntax to AST
Set curl at both ends of single-segment path
Last x solved for is d2 of last segment
Add Show instances to MetaFont types
MetaFont Parser: Handle & joins between paths
MetaFont: split solveOpen into solver & segment-rewriter
Handle 1-segment paths