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ARROW-4717 [C#] Consider exposing ValueTask instead of Task

Eliminating allocations in the C# Arrow library:

1. Use ValueTask instead of Task in the public APIs.
2. Eliminate allocations in ArrayData constructor by taking an array directly instead of calling `ToArray()` every time.
3. Remove some other minor allocations that provide no benefit.
4. Also, add a simple `NullCount == 0` check to optimize the case where there are no nulls in the data, but the null bitmap is filled with 0s instead of being empty.

With this change the `ArrowReaderWithMemory` benchmark in the repo goes from:

## Before
|                      Method |      Allocated Memory/Op |
|---------------------------- |--------------------:|
|       ArrowReaderWithMemory |  5.84 KB |

## After
|                      Method |      Allocated Memory/Op |
|---------------------------- |--------------------:|
|       ArrowReaderWithMemory |  4.48 KB |

@stephentoub @pgovind @chutchinson

Author: Eric Erhardt <eric.erhardt@microsoft.com>

Closes #4194 from eerhardt/ValueTask and squashes the following commits:

d2a00afd9 <Eric Erhardt> PR feedback.
310bb2f3b <Eric Erhardt> Convert all Task usages to ValueTask to save on allocations.
d1d2687d5 <Eric Erhardt> Minor optimizations