feat[lang]: allow hex literals for unsigned integer types

[tserg]

What I did

Resolves #4344.

How I did it

Add Hex nodes as valid literals for integers.

How to verify it

See tests.

Commit message

feat[lang]: allow hex literals for unsigned integer types

This commit adds support for hex values as unsigned integers 
by adding Hex nodes as valid literals for integers. Additional
typechecking is added to ensure that these hex values are 
of the same case, and that they cannot be used for signed
integers.

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.61%. Comparing base (794022e) to head (97feac2).

Files with missing lines	Patch %	Lines
vyper/codegen/expr.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4387      +/-   ##
==========================================
- Coverage   91.31%   88.61%   -2.71%     
==========================================
  Files         113      113              
  Lines       16061    16078      +17     
  Branches     2703     2708       +5     
==========================================
- Hits        14666    14247     -419     
- Misses        964     1314     +350     
- Partials      431      517      +86     

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[charles-cooper]

maybe out-of-scope but it would be much better for validate_expected_type to return the type it inferred

[tserg]

I think I/we have tried this a few times before without completing it 😄

[charles-cooper]

might be ok to allow this actually

[tserg]

hm if we allow hex literals for unsigned and signed ints, should we disallow folding involving any hex integer? since we do not know at the time of folding if a hex value should be interpreted as a signed integer or an unsigned integer

[tserg]

but I think excluding hex literals during folding would not work either because it would break things downstream where we expect a literal value during typechecking

[tserg]

what if we make the syntax more explicit like native hex bytes? e,g, unsigned hex integers would be u"0xff" and signed hex integers would be i"0xff".