Improve compatibility checker

[simoncozens]

This PR:

• adds checking for the compatibility of GPOS contexts between contextual anchors (which can cause very confusing merge errors as entire lookups end up not aligning).
• improves the display of compatibility error messages

Before:

ERROR:fontmake.compatibility:Fonts had differing point type in glyph D, contour 0, point 9:
ERROR:fontmake.compatibility: * Incompatible Sans Regular had curve
ERROR:fontmake.compatibility: * Incompatible Sans Bold had line
ERROR:fontmake.compatibility:Fonts had differing point type in glyph D, contour 0, point 10:
ERROR:fontmake.compatibility: * Incompatible Sans Regular had None
ERROR:fontmake.compatibility: * Incompatible Sans Bold had line
ERROR:fontmake.compatibility:Fonts had differing point type in glyph D, contour 0, point 11:
ERROR:fontmake.compatibility: * Incompatible Sans Regular had None
ERROR:fontmake.compatibility: * Incompatible Sans Bold had line

After:

ERROR:fontmake.compatibility:
Fonts had differing point type in glyph D, contour 0, point 9:
 * Incompatible Sans Regular had: curve
 * Incompatible Sans Bold had: line

ERROR:fontmake.compatibility:
Fonts had differing point type in glyph D, contour 0, point 10:
 * Incompatible Sans Regular had: None
 * Incompatible Sans Bold had: line

ERROR:fontmake.compatibility:
Fonts had differing point type in glyph D, contour 0, point 11:
 * Incompatible Sans Regular had: None
 * Incompatible Sans Bold had: line

[anthrotype]

LGTM thanks

[arrowtype]

@simoncozens THANK YOU for the readability upgrade here! This is a major UX improvement. Well done!