Implement `F_C_STRING` to convert a Fortran string to a C null-terminated string. Documented in F2023 Standard: 18.2.3.9 `F_C_STRING (STRING [, ASIS])`.
Implement `F_C_STRING` to convert a Fortran string to a C null-terminated string. Documented in F2023 Standard: 18.2.3.9 `F_C_STRING (STRING [, ASIS])`.