Simplify the function bool2str in main.f90

[Beliavsky]

It can be written more concisely as

    function bool2str(bool) result(str)
        logical, intent(in) :: bool
        character(len=:), allocatable :: str
        if (bool) then
            str = '.true.'
        else
            str = '.false.'
        end if
    end function bool2str

With gfortran the output of the code

module m
implicit none
contains
    function bool2str(bool) result(str)
        logical, intent(in) :: bool
        character(len=:), allocatable :: str
        if (bool) then
            str = '.true.'
        else
            str = '.false.'
        end if
    end function bool2str
end module m

program main
use m
implicit none
print "(a)", "'" // bool2str(.true.) // "'"
print "(a)", "'" // bool2str(.false.) // "'"
end program main

is

'.true.'
'.false.'