Rewrite internals and add new APIs and deprecation warnings.

[lilith]

This release makes user-facing changes with deprecation warnings. Please review your build warnings to avoid breakage in the future.

• There are new classes for attaching source image data to jobs; use MemorySource.* over ByteSource and BufferedStreamSource.* instead of StreamSource.
• Microsoft.IO.RecyclableMemoryStream 3.x is now required
• System.Buffers and System.Memory 4.x+ are now required on .NET 4.x / .NET Standard 2.0
• InputWatermark.Source is now IMemorySource instead of IBytesSource

It also makes lots of internal changes to increase performance, eliminate unnecessary allocations/copies, and improve compatibility with AOT and trimming.

It is now possible to provide ReadOnlyMemory data and IOwnedMemory data, without copying to a byte[] array. The new IAsyncMemorySource interface allows for asynchronous data sources, and the new IMemorySource interface allows for synchronous data sources.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 41.27907% with 404 lines in your changes are missing coverage. Please review.

Project coverage is 46.70%. Comparing base (ae68b97) to head (6cbbb6b).

Files	Patch %	Lines
src/Imageflow/Bindings/JobContext.cs	50.84%	53 Missing and 34 partials ⚠️
...c/Imageflow/Fluent/IOutputDestinationExtensions.cs	10.63%	41 Missing and 1 partial ⚠️
src/Imageflow/Fluent/MemorySource.cs	7.89%	32 Missing and 3 partials ⚠️
src/Imageflow/Fluent/BufferedStreamSource.cs	49.09%	19 Missing and 9 partials ⚠️
src/Imageflow/Fluent/ImageJob.cs	56.25%	25 Missing and 3 partials ⚠️
...nternal.Helpers/StreamMemoryExtensionsPolyfills.cs	32.25%	20 Missing and 1 partial ⚠️
src/Imageflow/Bindings/JsonResponse.cs	42.85%	19 Missing and 1 partial ⚠️
...ageflow/Internal.Helpers/UnmanagedMemoryManager.cs	5.00%	19 Missing ⚠️
src/Imageflow/Fluent/BytesDestination.cs	38.46%	11 Missing and 5 partials ⚠️
src/Imageflow/Fluent/StreamDestination.cs	0.00%	14 Missing ⚠️
... and 16 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
- Coverage   50.64%   46.70%   -3.95%     
==========================================
  Files          42       55      +13     
  Lines        2235     2715     +480     
  Branches      385      484      +99     
==========================================
+ Hits         1132     1268     +136     
- Misses        910     1212     +302     
- Partials      193      235      +42     

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