Rule requiring implicit none in interfaces is too strict

[LiamPattinson]

The reason interface blocks don't inherit the implicit none of their surrounding scope is that they're often used to create modular interfaces to old FORTRAN 77 code, and there's no way to tell in general which interface functions/subroutines refer to implicitly typed code and which refer to explicitly typed code.

A good candidate for where we should be asserting the use of implicit none in interface blocks is for bind(C) functions/subroutines.