Overloading of concatenation and assignment operators for character variables

[awvwgk]

Additionally to the basic character manipulation in stdlib_ascii and the stdlib_string_type we might want to allow for a somewhat looser and less strict string handling.

The idea would be to overload common string operators like // to allow concatenating with implicit conversion to character values.

Originally posted by @urbanjost in #336 (comment):

I find overloading helps make things much more compact, so assuming you have a function like STR, overloading // with that allows you to do things like "MESSAGE='the value is '//10//' and the limit is '//300.4//'.'". Some would probably like to overload + to do that, but I prefer to overload + - / * to convert strings to numbers, and to overload INT(), REAL(), and DBLE() to take character values and assume they are numeric. So that makes for a symmetric set for converting to and from strings and numeric values.

Originally posted by @ivan-pi in #336 (comment):

Both Java and Javascript support the idiom of using the concatenation operator to build a string using

String s = "" + i;

If operator(//) were overload then one could do the same thing in Fortran:

character(len=:), allocatable :: s
s = ''//42   ! or 42//''

Similarly, one could think about overloading the assignment operator for such purposes, at the risk of running into the usual fixed length vs. deferred length character issues.