define coercion types, value/impl types better
define coercion types, value/impl types better

We were using two different meanings of "value type", so now
value vs implementation type is "of" type vs "container" type.

We also now have a coercive type syntax, so there's no longer
any need for a separate 'as' trait for subs.

prammer/specs,tokuhirom/specs,tokuhirom/specs,perl6/specs,prammer/specs,perl6/specs,prammer/specs,tokuhirom/specs