Type coercion from function item types to function pointers

[mattheww]

The "Type coercions" section says coercion is allowed from:

• Function item types to fn pointers
• Non capturing closures to fn pointers

This ought to describe the compatibility requirements, including:

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

See ferrocene/specification#460 and ferrocene/specification#472