Remove unsafe code from DCS utility classes (UniqueId, Base64Encoding, XmlBufferReader)

Replaces fixed/byte*/char* and Unsafe.* pointer patterns with Span<T> and BinaryPrimitives in the standalone utility classes of System.Private.DataContractSerialization. The 4-char chunk loops in Base64Encoding and the 16-byte GUID parse/encode in UniqueId match the loop shapes that recent JIT BCE improvements eliminate.

No behavior change. Tests: System.Runtime.Serialization.Xml (354) + Json (173) all pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EgorBo