Remove implicit string operator from Utf8String

[am11]

No description provided.

[MichalStrehovsky]

This doesn't remove the conversions. I'm actually already working on removing the conversions (pushed out what I have at #122160). This PR will just introduce unnecessary merge conflicts.

[am11]

This doesn't remove the conversions.

This PR is improving a few places where we were accidentally allocating (calling .ToString() on Utf8String then assigning to Utf8String..) and it isn't regressing anything. It's good to keep allocations explicit to avoid such accidents.

[am11]

@MichalStrehovsky, merged and updated object writer.

[MichalStrehovsky]

@MichalStrehovsky, merged and updated object writer.

Thank you!

Could you do a pass over these and look at places that do undesirable allocations?

The places that currently use the implicit conversion operator are markers for "unreviewed legacy code". They may or may not be causing excessive allocations and conversions. The places are easy to search for. The places that use the string/byte[] constructors right now are "reviewed". They are known not to cause excessive allocations and we're not going to revisit them as part of the UTF-8 effort anymore.

This PR switches everything to use the string or byte[] constructors. This shakes things up and we can no longer distinguish between things that are done and we're happy with vs things that are not done yet. My preference would be to do this piecemeal and delete the uses of the operator in batches. However if you would like to delete the operator in this PR, I'd like to ask you to go over the places that do excessive allocations and fix them up so that we can maintain the "string or byte[] constructor means we're done here" invariant.

[am11]

@MichalStrehovsky, I’ve reviewed the changes and applied updates in 0fb2c78. If you notice anything that seems off, please let me know. I’ve intentionally left expressions such as Type.Name alone, because these values might include multi-byte characters.

[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop

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Azure Pipelines successfully started running 1 pipeline(s).

[MichalStrehovsky]

/ba-g I think the globalization issue has a fix in #122480