strings: join, to_c_char

[perazz]

This PR is spun off #911 for clarity.

Add two string utilities:

join

result = join(strings [, separator])  

• Joins an array of strings into a single string, using the specified separator
• Separator is optional, may have length>1 (default: 1 space).
• accepts character(*) or type(string_type) arrays.

This simplifies string concatenation for command construction and formatted output.

to_c_char

result = to_c_char(value)  

Converts a Fortran string to a null-terminated C character array (c_char), enabling interoperability with C libraries that require C-style strings.

• accepts character(*) or type(string_type) variables.
• name choice: to be similar to to_string. maybe we should consider to_c_char? or c_char to mimic the internal char interface?

cc: @jvdp1 @jalvesz @fortran-lang/stdlib

[jvdp1]

Nice. Thank you @perazz

• name choice: to be similar to to_string. maybe we should consider to_c_char? or c_char to mimic the internal char interface?

I prefer to_c_char over to_string. c_char could be an option too.

I will review the PR ASAP, but looks already good to me.

[jalvesz]

maybe we should consider to_c_char? or c_char to mimic the internal char interface?

I think to_c_char is a very good option and its clear about the intent of the procedure. c_char being the kind designator it would be confusing.

[jvdp1]

LGTM! Thank you @perazz

[perazz]

That was fast, thank you @jvdp1 @jalvesz!

I've renamed to to_c_char. As Jose points out, c_char would collapse with the integer kind, that can easily lead to errors

[jalvesz]

I saw in several places that you use cmd to refer to the output of the string manipulation. Would it be possible to use one of res, out or str ? I think cmd signals an explicit use of commands, but these implementations are much more generic than that.

[jalvesz]

LGTM @perazz! Great addition, no further comments on my side :)

[perazz]

Thank you for the reviews @jvdp1 @jalvesz, let's wait another day or two and then merge, if there are no further comments.