Improve string marshalling performance by using performant .NET APIs

[Perksey]

The following discussion from #505 should be resolved:

Please forward to Utf8.FromUtf16 on >= netcore3.0
Something like

#if NET5_0
Debug.Assert(Utf8.FromUtf16(input, span, out _, out convertedBytes, true, true) == OperationStatus.Done);
#endif

We probably don't care for the returned status, it can only be done, or theoretically, DestinationTooSmall but that can only happen if GetMaxSizeOf fails.
(If you don't think this is fit for this PR, do just reference this in a new issue, and I'll get back to it asap, we absolutely need to track this though, perf of the string marshalling is miserable)

[Perksey]

This isn't needed.

• SilkMarshal's use of GetBytes with pointers: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Text/UTF8Encoding.cs,342
• Calls into GetBytesCommon: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Text/UTF8Encoding.cs,391
• Which calls into GetBytesFast: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Text/UTF8Encoding.cs,420
• Which calls into Utf8Utility.TranscodeToUtf8: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Text/Unicode/Utf8Utility.Transcoding.cs,838
• Which Utf8.FromUtf16 also calls into: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Text/Unicode/Utf8.cs,64

Given that we've validated everything, these code paths are the same (FOR THIS SPECIFIC USE CASE) and there is limited benefit to be had here. Closing.