Support constants on class-string<Foo>

Release 2.14.0 has introduced a regression (#189) and has been replaced with 2.14.1. Original release notes follow:

• Support $foo::BAR where $foo is declared as class-string<Foo>, this partly fixes a very old issue (#186, thanks @eithed)

Internal change:

• Leading namespace separator is now removed consistently in one place (\Name\Space → Name\Space) (#187)