Support initial procedure pointer (`=>target`) declaration under F2008

[perazz]

fparser rejects valid Fortran 2008 procedure declaration when a procedure pointer is given an initial non-null target (null target is supported):

procedure(foo_cb), pointer, nopass :: eval => dummy_output

Parsing this with std='f2008' raises a FortranSyntaxError, while gfortran -std=f2008 runs it fine.

I have proposed a fix at #502. I found one reference of why that should be supported part of the F2008 language specs at J3 working document 10-007r1 §12.4.3.6:

R1214  proc-decl           is procedure-entity-name [ => proc-pointer-init ]
R1216  proc-pointer-init   is null-init
                           or initial-proc-target
R1217  initial-proc-target is procedure-name
C1219 (R1214) If => appears in proc-decl, the procedure entity shall
              have the POINTER attribute.
F2003 R1214 was procedure-entity-name [ => null-init ] - same rule number, narrower body. C1219 is unchanged between standards.

Thank you for the excellent work on fparser,
Federico