ice40_wrapcarry - iCE40: wrap carries
Wrap manually instantiated SB_CARRY cells, along with their associated SB_LUT4s,
into an internal $__ICE40_CARRY_WRAPPER cell for preservation across technology
Attributes on both cells will have their names prefixed with 'SB_CARRY.' or
'SB_LUT4.' and attached to the wrapping cell.
A (* keep *) attribute on either cell will be logically OR-ed together.
unwrap $__ICE40_CARRY_WRAPPER cells back into SB_CARRYs and SB_LUT4s,
including restoring their attributes.