refactor: merge annotation.py and local.py

[tserg]

What I did

Annotation at the module level is performed simultaneously with typechecking in vyper/semantics/analysis/module.py while annotation at the statement and expression level is performed in vyper/semantics/analysis/annotation.py with typechecking in vyper/semantics/analysis/local.py.

This PR merges annotation of statements and expressions into vyper/semantics/analysis/local.py for consistency with how it is done for modules, and makes it easier for other possible changes (e.g. moving some folding to after semantics analysis).

How I did it

Copy, paste and edit.

How to verify it

See tests.

Commit message

This commit merges two phases of analysis: typechecking
(`vyper/semantics/analysis/local.py`) and annotation of ast nodes with
types (`vyper/semantics/analysis/annotation.py`). This is both for
consistency with how it is done for `analysis/module.py`, and also
because it increases internal consistency, as some typechecking was
being done in `annotation.py`. It is also easier to maintain this way,
since bugfixes or modifications to typechecking need to only be done in
one place, rather than two passes. Lastly, it also probably improves
performance, because it collapses two passes into one (and calls
`get_*_type_from_node` less often).

This commit also fixes a bug with accessing the iterator when the
iterator is an empty list literal.

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #3456 (54c1908) into master (5482bbc) will decrease coverage by 0.07%.
The diff coverage is 96.93%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff             @@
##           master    #3456      +/-   ##
==========================================
- Coverage   89.08%   89.01%   -0.07%     
==========================================
  Files          86       85       -1     
  Lines       11455    11364      -91     
  Branches     2605     2585      -20     
==========================================
- Hits        10205    10116      -89     
- Misses        830      834       +4     
+ Partials      420      414       -6     

Files	Coverage Δ
vyper/builtins/_signatures.py	97.64% <100.00%> (ø)
vyper/builtins/functions.py	90.31% <100.00%> (-0.29%)	⬇️
vyper/semantics/analysis/common.py	100.00% <100.00%> (ø)
vyper/semantics/analysis/utils.py	92.04% <100.00%> (+0.07%)	⬆️
vyper/semantics/analysis/local.py	93.36% <96.74%> (+1.67%)	⬆️

... and 2 files with indirect coverage changes

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[tserg]

Some comments before I forget

[tserg]

The mypy lints are a bit ugly, and we cannot do file level ignores yet because we are stuck on mypy 0.910 due to py-evm. Is it okay to remove the type annotations for _LocalExpressionVisitor instead?

[tserg]

infer_arg_types of Len builtin was returning a python tuple of types.

[charles-cooper]

pushed some final changes/polishing. nice improvement to the typechecking code, great work!