Unsigned types improvements

[WojciechMazur]

• Introduce small unsigned integer caching for unsigned types (UByte, UShort, UInt, ULong, USize)
• Use caching for .toUnsigned conversions
• Add intrinsic unsignedOf operation - ensures a given value would be directly wrapped into Op.Box instead of multiple method calls (2xOp.Module + 2xOp.Method + 2xOp.Calls) we now emit single Op.Box. It would be used to implement toUnsigned conversion
• Implement toUnsigned operations as inlined extensions in Scala3. Keep the UnsignedRich* implicit classes in shared scala 2/3 sources, as they have lower priority, but allows to keep cross building source-compatiblity if importing each implicit class directly