[patch, libfortran] Fortran 2018: Support d0.d, e0.d, es0.d, en0.d, g0.d and ew.d e0 edit descriptors

[Jerry]

Hi all,

The attached patch includes adjustments to the test case.

The Fortran Standard states the exponent width when using the e0 
exponent specfier results in the smallest possible exponent width.  This 
patch implements that case.

I got frustrated with trying to re-understand this code segment and even 
found some dead code in there.  As a result I did some major refactoring 
of the code and separated out the zero width, positive width, and no 
width DEC extensions into their own chunks.  I also added comments in 
hopes of helping others follow what this is doing and how it works.

This patch resolves some parsing issues currently on trunk where a 
format specifier following the e0.d.e specifier would result in an error 
(comments 12 and 20 of the PR). These are fixed.

The patch, as it is, passes regression testing but I must confess I may 
not have all the DEC stuff right yet and I propose we commit the patch 
here and address any DEC stuff as a follow up. (I will be looking at the 
DEC stuff in the next few days.)

OK for trunk?

Regards,

Jerry
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