This is essentially KevinCathcart's work, with a test suite and changes required to ensure that these tests pass.

Also removed other unused "using" statements.

Removed use of string.CompareOrdinal when processing part pieces.
Replaced byte allocations with ArrayPool Rent/Returns.
Made InterleavedZipPackagePartStream use Spans instead of byte arrays. This has increased memory usage when .NET Framework calls Read(byte[], int, int) and Write(byte[], int, int).

Split Read and ReadCore apart, so that a new Read(Span<byte>) method doesn't appear on the .NET Standard 2.0 path. Also adjusted the conditions from "NETCOREAPP2_1_OR_GREATER" to "!NETFRAMEWORK && !NETSTANDARD2_0", to align with the rest of .NET.

…poses

I can't see a simple way to do this in the Write() path - we rent and return across some loop iterations

* Corrections to comments and spacing.
* Switched to the use of an EndOfStreamException in the correct situation.
* Adjusted the SeekUnderlyingPieceStream method to slightly tweak the mechanism and to make use of a readBuffer which might be larger than BufferSize.
* Small refactor of part piece deletion.

This was only used to reference ZipPackagePartPiece.TryParse and .Index, so switched the tests to use reflection.

* Using exception helpers for .NET
* Removing #regions
* Fixing nullability in references to ZipPackagePartPiece.PieceDescriptors
* Spacing/spelling/grammar changes to comments

Replaced references to NETFRAMEWORK and NETSTANDARD2_0 with NET and !NET as appropriate.