Expand LZ4Pickler API to enable better perf & composability.

[geeknoid]

• Introduce UnpickledSize which returns the expected size of a pickled
  item once unpickled.

• Introduce an Unpickle overload which takes a Span<byte> and one that
  takes an IBufferWriter for output. Both of these enable the caller to
  reuse buffers across multiple unpicklings, which increases perf and
  reduces pressure on the GC.

• Introduce a Pickle overload which takes an IBufferWriter as output.
  Using this overload enables the caller to reuse buffers, and eliminates
  a full data copy by unpickling directly into the final destination
  buffer.

• The code was rewritten to mostly be safe by leveraging Span<T>. There
  is only a teeny bit of unsafe code left in the pickling path.

[MiloszKrajewski]

Hi,

Sorry for the delay, I just want to dedicate it a little bit more time that I have this weekend. I'll merge it next week.

Thanks,

[geeknoid]

Hi, anything I can do to make this easier to merge?

Thanks.

[MiloszKrajewski]

Hi,
I made mess in git, as usual, git is just designed to be easily messed.
I merged it into master (I probably shouldn't). Unfortunately it was not compiling, so I reverted your pull request to keep master clean. I have your changes 'on a side' so I will take a look and reapply, but I definitely expected it to be less troublesome.
Work in progress.

[geeknoid]

I'm sorry this is causing problems. I was not able to get your original build system working on my end, so I hacked something together enough so that I could run your unit tests and show that the code itself was functional.

Please let me know if I can help get this working fully.