Allow uint256 as iterator type in range-based for loop

[iamdefinitelyahuman]

What I did

Allow use of types other than int128 for range-based for loops.

Fixes #2153
Fixes #2185

How I did it

This was already made possible in the type checking pass, but it gets blocked in parser.stmt. The proper solution is to refactor parser, but that is still potentially months away. As a stopgap solution:

• attach type information to the iterator target AST node during type checking.
• when generating LLL, use the given type instead of assuming int128.
• if no type information is attached (happens when parser generates smaller ASTs later in the compilation phase, without any type checking), default to int128

How to verify it

Run tests.

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[codecov-commenter]

Codecov Report

Merging #2180 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2180   +/-   ##
=======================================
  Coverage   85.32%   85.33%           
=======================================
  Files          83       83           
  Lines        8403     8405    +2     
  Branches     2032     2032           
=======================================
+ Hits         7170     7172    +2     
  Misses        731      731           
  Partials      502      502           

Impacted Files	Coverage Δ
vyper/ast/nodes.py	93.84% <100.00%> (ø)
vyper/context/validation/local.py	88.42% <100.00%> (+0.04%)	⬆️
vyper/parser/stmt.py	79.79% <100.00%> (+0.06%)	⬆️

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[fubuloubu]

Add a few tests and g2g

[codecov-io]

Codecov Report

Merging #2180 into master will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2180      +/-   ##
==========================================
- Coverage   85.32%   85.28%   -0.05%     
==========================================
  Files          83       79       -4     
  Lines        8403     8221     -182     
  Branches     2032     2005      -27     
==========================================
- Hits         7170     7011     -159     
+ Misses        731      711      -20     
+ Partials      502      499       -3     

Impacted Files	Coverage Δ
vyper/ast/nodes.py	93.84% <100.00%> (ø)
vyper/context/validation/local.py	90.08% <100.00%> (+1.70%)	⬆️
vyper/parser/stmt.py	79.79% <100.00%> (+0.06%)	⬆️
vyper/parser/parser_utils.py	80.49% <0.00%> (-0.16%)	⬇️
vyper/opcodes.py
vyper/exceptions.py
vyper/typing.py
vyper/__main__.py

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[fubuloubu]

Why'd coverage go down so much here?

EDIT: premature update from codecov

[iamdefinitelyahuman]

There is a very minor loss to coverage in vyper/parser/parser_utils.py. I think it's that failing paths are no longer being hit, since i is no longer required to be int128.