Implement proper deallocation

[certik]

Of:

• dicts
• lists
• strings (Implement proper string deallocation in the LLVM backend #805)
• function return values (How to handle deallocatable function return values #494)

And anything else that might leak memory currently.

[czgdp1807]

The deallocation should be implicit (done by LPython), explicit (done by user via del statements), right?

[certik]

Yes, implicit when the (allocatable) variable goes out of scope. I think dicts, lists, strings should have "allocatable" set, and then we should add some mechanism into libasr that both LFortran and LPython can use to automatically insert the "ImplicitDeallocate" ASR nodes.

And del would delete it right away explicitly, just like deallocate in LFortran.