enforce type parameter constraint equality for core API types

git-svn-id: http://haxe.googlecode.com/svn/trunk@6607 f16182fa-f095-11de-8f43-4547254af6c6
refined String.substr/substring specification

git-svn-id: http://haxe.googlecode.com/svn/trunk@6608 f16182fa-f095-11de-8f43-4547254af6c6