Fast type checking for heap types

[encukou]

Type-checking -- specifically, checking whether a given type has a particular memory layout -- is currently hard to do for heap types.

[gvanrossum]

@encukou Can you be more specific about what this would look like? How would the "particular memory layout" be specified?

[encukou]

A solution is fully sketched out in encukou/abi3#19, my next step is writing the implementation and PEP.

[vstinner]

Is it an API issue? To me, it looks more like an implementation issue, no?

[encukou]

I don't know what you mean by those. Does it matter?