It would be useful to have a stdlib module for handling common file format for d…ata storage. If we can provide an interface in stdlib for this kind of operations we would make it much easier to integrate a new file format in a library which is already using stdlib.
This can be implemented in stdlib by pulling in common libraries used for this task, like JSON Fortran and TOML Fortran. Alternative, the interface could be declared in the specification and libraries like JSON Fortran and TOML Fortran can provide an implementation of `stdlib_json` or `stdlib_toml`, respectively.
**Requirements** (feel free to add to this list)
- [x] map data structure
- [ ] list data structure
- [ ] date time derived type (for TOML)
<details><summary>A possible API</summary>
```f90
!> Standard library support for reading TOML documents
module stdlib_toml
interface toml_load
!> Load TOML data structure from file
module subroutine toml_load_from_file(filename, value, stat, message)
!> File name to read from
character(*), intent(in) :: filename
!> Data structure, either a built-in type a [[stdlib_value::value_type]]
class(*), allocatable, intent(out) :: value
!> Error code, if not provided routine will halt in case of error
integer, intent(out), optional :: stat
!> Error message
character(:), allocatable, intent(out), optional :: message
end subroutine toml_load_from_file
end interface toml_load
interface toml_loads
module subroutine toml_load_from_string(string, value, stat, message)
!> File name to read from
character(*), intent(in) :: string
!> Data structure, either a built-in type or a [[stdlib_value::value_type]]
class(*), allocatable, intent(out) :: value
!> Error code, if not provided routine will halt in case of error
integer, intent(out), optional :: stat
!> Error message
character(:), allocatable, intent(out), optional :: message
end subroutine toml_load_from_string
end interface toml_loads
end module stdlib_toml
```
A similar interface would be provided for reading JSON to allow a consistent interface for all common IO formats.
```f90
!> Standard library support for reading JSON documents
module stdlib_json
interface json_load
!> Load JSON data structure from file
module subroutine json_load_from_file(filename, value, stat, message)
!> File name to read from
character(*), intent(in) :: filename
!> Data structure, either a built-in type a [[stdlib_value::value_type]]
class(*), allocatable, intent(out) :: value
!> Error code, if not provided routine will halt in case of error
integer, intent(out), optional :: stat
!> Error message
character(:), allocatable, intent(out), optional :: message
end subroutine json_load_from_file
end interface json_load
interface json_load
!> Load JSON data structure from string
module subroutine json_loads_from_string(string, value, stat, message)
!> File name to read from
character(*), intent(in) :: string
!> Data structure, either a built-in type or a [[stdlib_value::value_type]]
class(*), allocatable, intent(out) :: value
!> Error code, if not provided routine will halt in case of error
integer, intent(out), optional :: stat
!> Error message
character(:), allocatable, intent(out), optional :: message
end subroutine json_loads
end interface json_loads_from_string
end module stdlib_json
```
</details>
