This PR fixes#125446 by specializing the error message that is
generated when an arbitrary expression is used as an IO Unit. See the
tests for specific examples, but the general gist is that if you use a
non-variable expression as the IO unit argument to a read or write, you
now get a more specific error message indicating the the expression
could have been a scalar integer expression or character variable.
Implement the UNSIGNED extension type and operations under control of a
language feature flag (-funsigned).
This is nearly identical to the UNSIGNED feature that has been available
in Sun Fortran for years, and now implemented in GNU Fortran for
gfortran 15, and proposed for ISO standardization in J3/24-116.txt.
See the new documentation for details; but in short, this is C's
unsigned type, with guaranteed modular arithmetic for +, -, and *, and
the related transformational intrinsic functions SUM & al.
