Streamlined ABI encoder logic for returning tuple types

[iamdefinitelyahuman]

What I did

Simplify the logic around returning tuples, when each member of the tuple is of the same type.

How I did it

Add a special if branch that replicates the logic for returning an array type. This branch is only reached if every member of the tuple has the same type, and that type is a base type (or array of the base type).

This has a not-insignificant effect on the bytecode size.

How to verify it

Run tests (TODO)

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[codecov-io]

Codecov Report

Merging #2302 (0605abb) into master (27e8e66) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2302      +/-   ##
==========================================
- Coverage   85.84%   85.81%   -0.04%     
==========================================
  Files          85       85              
  Lines        8539     8556      +17     
  Branches     2034     2040       +6     
==========================================
+ Hits         7330     7342      +12     
- Misses        727      730       +3     
- Partials      482      484       +2     

Impacted Files	Coverage Δ
vyper/codegen/return_.py	100.00% <100.00%> (ø)
vyper/functions/functions.py	89.25% <0.00%> (-0.57%)	⬇️

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[fubuloubu]

Add a few tests