feat: add utility functions to OrderedSet

[harkal]

What I did

Added support for:

1. Convenience method to get the first member
2. Computing the intersection operation on any number of sets

How I did it

How to verify it

Commit message

Add utility methods to OrderedSet()

• first()
• intersection() (class method)

Description for the changelog

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[charles-cooper]

Do you need to explicitly implement iter? Seems like it should inherit from dict

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 28.57143% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 85.99%. Comparing base (8d34e74) to head (a0033bc).

Files	Patch %	Lines
vyper/utils.py	28.57%	10 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3833      +/-   ##
==========================================
- Coverage   86.10%   85.99%   -0.12%     
==========================================
  Files          92       92              
  Lines       13821    13833      +12     
  Branches     3052     3056       +4     
==========================================
- Hits        11901    11895       -6     
- Misses       1482     1500      +18     
  Partials      438      438              

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[harkal]

Do you need to explicitly implement iter? Seems like it should inherit from dict

You are right, we don't... :/