feat: require type annotations for loop variables

[tserg]

What I did

Fix #3475, #3441

How I did it

How to verify it

Commit message

feat: require type annotations for loop variables

this commit changes the vyper language to require type annotations for
loop variables. that is, before, the following was allowed:
```vyper
for i in [1, 2, 3]:
    pass
```

now, `i` is required to have a type annotation:
```vyper
for i: uint256 in [1, 2, 3]:
    pass
```

this makes the annotation of loop variables consistent with the rest of
vyper (it was previously a special case, that loop variables did not
need to be annotated).

the approach taken in this commit is to add a pre-parsing step which
lifts out the type annotation into a separate data structure, and then
splices it back in during the post-processing steps in
`vyper/ast/parse.py`.

this commit also simplifies a lot of analysis regarding for loops.
notably, the possible types for the loop variable no longer needs to be
iterated over, we can just propagate the type provided by the user. for
this reason we also no longer need to use the typechecker speculation
machinery for inferring the type of the loop variable. however, the
NodeMetadata code is not removed because it might come in handy at a
later date.

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0c82d0b) 84.23% compared to head (9678f91) 84.25%.

Files	Patch %	Lines
vyper/ast/parse.py	91.30%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3596      +/-   ##
==========================================
+ Coverage   84.23%   84.25%   +0.02%     
==========================================
  Files          92       92              
  Lines       13034    13073      +39     
  Branches     2919     2916       -3     
==========================================
+ Hits        10979    11015      +36     
- Misses       1636     1640       +4     
+ Partials      419      418       -1     

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[charles-cooper]

i think instead of passing around loop_var_annotations everywhere we should just tag it on the AST during the annotate_vyper_ast portion of ast parsing.

[charles-cooper]

i guess since folding isn't performed any longer we can kind of skip the intermediate variable assignment (and rename the function!)

[charles-cooper]

i just reverted the change since it's not relevant to this PR

[charles-cooper]

i don't think we need to attach this into the metadata tbh. this should all be handled before analysis, and ideally before we even get to vyper AST construction.

[charles-cooper]

i think we can accomplish all this just with adding a visit_For into the visitor in this file

[charles-cooper]

nice work. thanks!