This PR fixes `not yet implemented: procedure pointer component in
structure constructor` as shown in the following test case.
```
MODULE M
TYPE :: DT
PROCEDURE(Fun), POINTER, NOPASS :: pp1
END TYPE
CONTAINS
INTEGER FUNCTION Fun(Arg)
INTEGER :: Arg
Fun = Arg
END FUNCTION
END MODULE
PROGRAM MAIN
USE M
IMPLICIT NONE
TYPE (DT) :: v2
PROCEDURE(FUN), POINTER :: pp2
v2 = DT(pp2)
v2 = DT(bar())
CONTAINS
FUNCTION BAR() RESULT(res)
PROCEDURE(FUN), POINTER :: res
END
END
```
This PR fixes a subset of procedure pointer component initialization in
structure constructor.
It covers
1. NULL()
2. procedure
For example:
```
MODULE M
TYPE :: DT
!PROCEDURE(Fun), POINTER, NOPASS :: pp1
PROCEDURE(Fun), POINTER :: pp1
END TYPE
CONTAINS
INTEGER FUNCTION Fun(Arg)
class(dt) :: arg
END FUNCTION
END MODULE
PROGRAM MAIN
USE M
IMPLICIT NONE
TYPE (DT), PARAMETER :: v1 = DT(NULL())
TYPE (DT) :: v2
v2 = DT(fun)
END
```
Passing a procedure pointer itself or reference to a function that
returns a procedure pointer is TODO.
