refactor: constant folding

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

refactor constant folding into a visitor class, clean up a couple passes

this moves responsibility for knowing how to fold a node off the
individual AST node implementations and into the ConstantFolder visitor.

by adding a dependency to get_namespace() it also makes constant folding
more generic; soon we can rely on more things being in the global
namespace at constant folding time.

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Attention: 39 lines in your changes are missing coverage. Please review.

Comparison is base (a1fd228) 84.21% compared to head (92fff06) 84.11%.

Files	Patch %	Lines
vyper/semantics/analysis/pre_typecheck.py	80.43%	14 Missing and 13 partials ⚠️
vyper/semantics/types/module.py	25.00%	6 Missing ⚠️
vyper/semantics/types/user.py	14.28%	6 Missing ⚠️

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3719      +/-   ##
==========================================
- Coverage   84.21%   84.11%   -0.11%     
==========================================
  Files          92       92              
  Lines       13077    13109      +32     
  Branches     2916     2921       +5     
==========================================
+ Hits        11013    11026      +13     
- Misses       1646     1663      +17     
- Partials      418      420       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tserg]

Two small comments, otherwise LGTM!