remove with_stateless and only_stateless. Replace with variants: [method,function]

[zdevito]

with_stateless: True === variants: [ method, function ]
only_stateless === variants: [ functions ]
nothing specified (i.e. default variants): variants: [ method ]

• do it for TensorLib, leaving with_stateless and only_stateless in place
• backport it to actual cwrap and remove with_stateless and only_stateless

[killeent]

This is done.