feat: implement lgt, llt, lle,lge

[HarshitaKalani]

Continuing: #3570
Towards: #3034

[HarshitaKalani]

program main
    character(5), allocatable :: a, b
    print*, lgt(maybe(a), b)

    contains
    function maybe(string) result(maybe_string)
        character(5), allocatable:: string
        character(len=len(string)) :: maybe_string
        maybe_string = 'works'
    end function maybe

end

$ lfortran a.f90
Segmentation fault (core dumped)

This does not work yet.

[Pranavchiku]

A small suggestion requested, also this PR needs to be rebased and conflicts resolved.

[Pranavchiku]

The failure seems unrelated and can be fixed by rerun

100% tests passed, 0 tests failed out of 3

Total Test time (real) =   9.93 sec
+ cd ..
+ git clean -dfx
Removing lf-fast/
Removing lf/
Removing model.dat
+ git checkout -t origin/namelist
fatal: 'origin/namelist' is not a commit and a branch 'namelist' cannot be created from it

[certik]

The failure is due to #3663.

fixed