fix[ux]: fold keccak and sha256 of constant hexbytes

[tserg]

What I did

We omitted to fold hexbytes for keccak and sha256 builtins.

The following example should compile but does not:

PROXY_CHILD_BYTECODE: constant(Bytes[16]) = x"67363d3d37363d34f03d5260086018f3"
PROXY_CHILD_BYTECODE_HASH: constant(bytes32) = keccak256(PROXY_CHILD_BYTECODE)

Error compiling: test.vy
vyper.exceptions.StateAccessViolation: Value must be a literal

  contract "test.vy:4", line 4:47 
       3 PROXY_CHILD_BYTECODE: constant(Bytes[16]) = x"67363d3d37363d34f03d5260086018f3"
  ---> 4 PROXY_CHILD_BYTECODE_HASH: constant(bytes32) = keccak256(PROXY_CHILD_BYTECODE)
  ------------------------------------------------------^
       5

How I did it

Handle HexBytes node when folding these builtins.

How to verify it

See new tests.

Commit message

This commit allows `keccak` and `sha256` of a constant hexbytes value
to be folded.

Description for the changelog

Fold keccak and sha256 of constant hexbytes.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.35%. Comparing base (61d259a) to head (68d0343).
Report is 87 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4536   +/-   ##
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  Coverage   92.35%   92.35%           
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  Files         123      123           
  Lines       17499    17499           
  Branches     2949     2949           
=======================================
  Hits        16162    16162           
  Misses        933      933           
  Partials      404      404           

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[charles-cooper]

lgtm. paging @cyberthirst for review