get rid of Core.Box in the package

[KristofferC]

Uses JuliaLang/julia#60478.

Tests fail locally for me so I'm not sure what state the repo is in right now...

[tecosaur]

The tests all passed on the last commit previously, it's strange to hear that seems to have ambiently changed

Test Summary: | Pass  Total  Time
SimpleColor   |   15     15  1.1s
Test Summary: | Pass  Total  Time
Faces         |   89     89  2.0s
Test Summary: | Pass  Total  Time
Styled Markup |   77     77  0.8s
Test Summary: | Pass  Total  Time
Fuzz (1000)   |    1      1  1.8s
Test Summary:     | Pass  Total  Time
AnnotatedIOBuffer |    5      5  0.0s
Test Summary: | Pass  Total  Time
ANSI encoding |   46     46  1.2s
Test Summary: | Pass  Total  Time
HTML encoding |   24     24  0.6s
Test Summary: | Pass  Total  Time
Legacy        |   22     22  0.2s
     Testing StyledStrings tests passed 

Edit: Oh the CI failures are purely codecov, the tests are passing in CI but not locally. Interesting...

[tecosaur]

From a read of the changes, I can't see any way this could impact behaviour, and tests are passing in CI, so I'm inclined to merge this and then investigate the local test failures.

I'll just add a comment for my reference pointing out why let key = key is used (and very slightly tweak the commit message).