[Patch, Fortran] Add coarray communication support to the trunk (coindex variables)

[Tobias Burnus]

This patch add the first coarray communication support to the trunk 
(ignoring the co_sum/co_min/co_max support, which was recently merged). 
[Note: In terms of the library this is still libcaf_single, but see below.]

The patch is based on my work on the fortran-caf branch, but has a 
slightly modified ABI. The patch should support most communications, but 
it is not complete. I intent to submit soon a patch which irons some 
wrinkles.

In particular, this patch adds three library calls to handle coindexed 
communication: Assignment to a coindex variable (caf_send), a coindexed 
expression (caf_expression) and assigning a coindexed variable to a 
coindexed variable (caf_sendget). The coarray is identified by a token 
(opaque object provided by the coarray library), an offset to that base 
address, an image index and an array descriptor for the coarray, which 
is also used for scalars – and which has the value of the whole array 
for vector subscripts. Additionally, one passes a "kind" variable as 
extra argument as the current array descriptor cannot destinguish a 
len=1 kind=4 from a len=4 kind=1 character string. And for vector 
subscripts, the subscripts are passed as additional argument.

For assignments, the library is supposed to handle padding/trimming of 
strings and type conversion (e.g. "cmplx_caf(:)[i] = int_array") as well 
as "array = scalar" assignments.

The following is left to be done as follow up:

* Support of vector subscripts with assumed-size variables: To be 
tested; might need the new array descriptor or some similar work around 
– or just a test case.
* The library libcaf_single supports padding/trimming of strings but 
still lacks the support for type conversion and vector subscripts.
* Adding an ABI documentation
* There are still some issues with regards to polymorphic coarrays, in 
particular with passing them as dummy arguments and in ASSOCIATE/SELECT 
TYPE, but presumably also with using them in coindexed expressions.

And as bigger item: Allocatable components of coarrays are not supported 
– not is the access to pointer or allocatable components (part refs); 
currently, there is no compile time diagnostic for it.


Additionally, I have remove the vector subscript preparations from the 
co_sum/min/max as it does not make much sense for those. And I added a 
collective test case, which I found on my hard disk.

Build and regtested. OK for the trunk?

Tobias

PS: Additional missing bits, not listed above: Locking and CRITICAL and 
atomics for Fortran 2008. And for TS18508 co_broadcast and co_reduce, 
the atomics extensions, teams, events and error recovery.
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