feat[lang]: allow downcasting of bytestrings

[charles-cooper]

What I did

How I did it

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Commit message

this commit extends `convert()` to allow downcasting of Bytes/Strings,
i.e. converting `Bytes[20]` to `Bytes[19]`.

this improves the UX of bytestrings somewhat, since currently (prior to
this commit) there is no type-safe way to decrease the size of a
bytestring in vyper. it also prepares us a little bit for adding generic
bytestrings inside the type system (`Bytes[...]`) which can only be
user-instantiated by `convert`ing to a known length.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 86.01%. Comparing base (cf37ec2) to head (5348802).

Files	Patch %	Lines
vyper/builtins/_convert.py	87.50%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3832      +/-   ##
==========================================
- Coverage   86.15%   86.01%   -0.14%     
==========================================
  Files          92       92              
  Lines       13923    13931       +8     
  Branches     3068     3071       +3     
==========================================
- Hits        11995    11983      -12     
- Misses       1491     1509      +18     
- Partials      437      439       +2     

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