keep SizeConstraints extensible

[riebl]

Fix #290, #383, #407 where APC_EXTENSIBLE is lost for SEQUENCE SIZE(, ...) OF

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.003%) to 70.916% when pulling c35ebd3 on riebl:fix_size_constraint into 00fa516 on vlm:master.

[mouse07410]

I like this PR, and it passes the CI - so I merged it into my fork.

Thanks, @riebl

[riebl]

As an avid user of your fork @mouse07410, I am happy to hear that you have merged my PR :-)