Remove CheckedPointer struct and replace its use with Span<T> instances

[h3xds1nz]

Description

Removes CheckedPointer which was used in quite some places back in early WPF implementations in NetFX but now the only thing using it is TrueTypeFontDriver and replaces it with ReadOnlySpan<T>.

• The Probe checks are the same as implemented by ReadOnlySpan<T> when slicing.
• Removes now useless Marshal.Copy with simple .ToArray() call from ReadOnlySpan<T> in ComputeFontSubset.
• I've used Slice and non-Try methods from BinaryPrimitives to be sure the exception behavior stays the same.
• Offers quite nice performance improvement over the original version.

SetFace method

Method	Mean [ns]	Error [ns]	StdDev [ns]	Gen0	Code Size [B]	Allocated [B]
Original	94.60 ns	1.523 ns	1.564 ns	0.0358	1,325 B	600 B
PR__EDIT	53.56 ns	1.039 ns	0.921 ns	0.0358	512 B	600 B

Customer Impact

Smaller code-size of PresentationCore, improved performance in this particular scenario.
For developers it introduces more familiar types in internal classes than custom implementations for a simple thing.

Regression

No.

Testing

Tested base font file parsing and class (TrueTypeFontDriver) functionality after CheckedPointer removal.

Risk

Low.

Microsoft Reviewers: Open in CodeFlow

[h3xds1nz]

Resolved merge conflicts 10