Add SFINAE helper for detecting attribute accessors
Add SFINAE helper for detecting attribute accessors

With constructors, we have accessors which do not correspond to an
attribute. Hence, we can not rely on a member `attr` to be present.

This commit adds a SFINAE helber which detects the presence/absence of
this member and checks whether the member is identical to a specific
attribute.

The utility will be used for enabling and disabling getters and setters
in the accessor bundle.

neithernut/cmoh,neithernut/cmoh