Bugfix: returning tuples from private functions

[iamdefinitelyahuman]

What I did

Fix a memory offset issue when returning tuple types from a private function.

Fixes #1847

How I did it

In parser/self_call.py:

• update the offset by the actual size of the value rather than 32
• ensure there is a new mstore for each value of an array inside a tuple.

How to verify it

Run tests. I added some new cases.

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #1968 into master will decrease coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1968      +/-   ##
==========================================
- Coverage   86.99%   86.57%   -0.42%     
==========================================
  Files          58       58              
  Lines        6629     6631       +2     
  Branches     1673     1674       +1     
==========================================
- Hits         5767     5741      -26     
- Misses        546      569      +23     
- Partials      316      321       +5     

Impacted Files	Coverage Δ
vyper/parser/self_call.py	94.44% <100.00%> (+0.10%)	⬆️
vyper/functions/functions.py	87.17% <0.00%> (-5.10%)	⬇️
vyper/ast/nodes.py	92.85% <0.00%> (-1.74%)	⬇️
vyper/parser/expr.py	86.98% <0.00%> (+0.43%)	⬆️
vyper/codegen/abi.py	66.79% <0.00%> (+3.39%)	⬆️

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[fubuloubu]

Good set of tests here, I would like to see more, but this solves the bug for now.