Replace calls to `Marshal.PtrToStringUTF8` methods with `Utf8StringMarshaller`

[jkotas]

I like this pattern of calling methods on Utf8StringMarshaller instead of Marshal...... I am wondering whether we should do find & replace of all Marshal.PtrToStringUTF8 and friends with this. I have noted it in #75630 (comment) .

Originally posted by @jkotas in #117635 (comment)

[jkotas]

@copilot

• Replace calls to Marshal.PtrToStringUTF8 and Marshal.StringToPtrUTF8 with calls of equivalent methods on Utf8StringMarshaller throughout the repo
• Delete unnecessary (IntPtr) casts, update the code to use byte* for the unmanaged string as appropriate
• Do not change interop tests.

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[huoyaoyuan]

Should we create documentation/guidance/auto suggestions for this? I've seen a lot of occurrences in existing codebases.

There are also other alternatives, Encoding.UTF8.GetString, new string(sbyte*) (uses ACP actually). We should decide recommendations for various scenarios, for example trimmer friendlyness.

[jkotas]

We are going to have guidance to use actual unmanaged pointer types for unmanaged pointers (instead of IntPtrs) as part fixing unsafe (dotnet/designs#330). cc @EgorBo