Folding builtins

[iamdefinitelyahuman]

What I did

Add constant folding logic for builtin functions.

How I did it

• folding logic is found in the evaluate method of each class in vyper/functions/functions.py
• the high level execution is handled via replace_builtin_functions in vyper/ast/folding.py
• test cases are in tests/functions/folding/

The only folding implementation I'm not testing is method_id. This function should always fold, so verifying the result of folding vs not-folding seems redundant. Once we add folding into the compiler process we can update the existing test cases around this function.

Next Steps

Things to do next that are outside the scope of this PR.

• add documentation
  • updates to "Types" and "Builtin functions" within RTD
  • add docstrings to classes in vyper/functions/functions.py
  • add README.md to vyper/functions
• plug folding logic into main compiler process
• refactor out now-redundant code

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #1949 into master will decrease coverage by 2.24%.
The diff coverage is 39.27%.

@@            Coverage Diff             @@
##           master    #1949      +/-   ##
==========================================
- Coverage   87.04%   84.79%   -2.25%     
==========================================
  Files          55       56       +1     
  Lines        6444     6642     +198     
  Branches     1629     1689      +60     
==========================================
+ Hits         5609     5632      +23     
- Misses        531      692     +161     
- Partials      304      318      +14     

Impacted Files	Coverage Δ
vyper/ast/__init__.py	100.00% <ø> (ø)
vyper/ast/validation.py	21.73% <21.73%> (ø)
vyper/functions/functions.py	77.58% <38.53%> (-19.62%)	⬇️
vyper/ast/folding.py	79.01% <66.66%> (-5.12%)	⬇️
vyper/exceptions.py	90.41% <100.00%> (ø)
vyper/ast/nodes.py	85.50% <0.00%> (-5.12%)	⬇️

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[fubuloubu]

Lol, looks like you found a Hypothesis bug

https://github.com/vyperlang/vyper/pull/1949/checks?check_run_id=651123760

[iamdefinitelyahuman]

@fubuloubu unless you have other comments I think this is ready for merging after #1956 merges.