refactor: Avoid casting away constness

Seems confusing and brittle to remove const and then add it back in the
return type.

src/span.h

@@ -270,7 +270,7 @@ Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
 inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; }
 inline unsigned char* UCharCast(unsigned char* c) { return c; }
 inline unsigned char* UCharCast(std::byte* c) { return (unsigned char*)c; }
-inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; }
+inline const unsigned char* UCharCast(const char* c) { return reinterpret_cast<const unsigned char*>(c); }
 inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
 inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }