Type coercion from function item types to function pointers

[mattheww]

§4.12.3:26 (fls_ulcdetwp6x96) says that type coercion is allowed to occur when

The source type is a function item type and the target type is a function pointer type.

This ought to describe the compatibilty requirements, including:

• the signatures must be compatible (which needs defining)
• the ABI must be the same
• the safety must be the same

Similarly for §4.12.3:27.