Use PooledByteBufferWriter to save 1K in trimmed System.Memory.dll

[steveharter]

Fixes #51571

Per that issue there was a ~1K regression.

Benchmark | Baseline | Test | Test/Base | Baseline IR | Compare IR | IR Ratio | Baseline ETL | Compare ETL
-- | -- | -- | -- | -- | -- | -- | -- | --
SOD - New Blazor Template - Publish - pub/wwwroot/_framework/System.Memory.dll.gz | 8.59 KB | 9.26 KB | 1.08
-- | -- | -- | --

Verified the ~1K savings locally (in self-contained trimmed console app):

Before: 35,840 bytes
After: 34,816 bytes

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Issue Details

---

Fixes #51571

Per that issue there was a ~1K regression.

Benchmark | Baseline | Test | Test/Base | Baseline IR | Compare IR | IR Ratio | Baseline ETL | Compare ETL
-- | -- | -- | -- | -- | -- | -- | -- | --
SOD - New Blazor Template - Publish - pub/wwwroot/_framework/System.Memory.dll.gz | 8.59 KB | 9.26 KB | 1.08
-- | -- | -- | --

Verified the ~1K savings locally (in self-contained trimmed console app):

Before: 35,840 bytes
After: 34,816 bytes

Author:	steveharter
Assignees:	steveharter
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

Nit: the outer using statement can be written with the using var syntax to avoid one degree of nestedness.