Mark UInt128 and other obvious readonly structs as readonly.

[eerhardt]

This is a follow-up to #1475. Using the in keyword with structs that aren't marked as readonly can cause perf issues due to hidden copies being made.

UInt128 is used in ValueMapper and potentially other places that may use the in keyword. Marking it as readonly, and while I was here - going through all struct declarations and marking the obvious ones as readonly as well.

[TomFinley]

Ha, nice thanks @eerhardt.... Though in principle we've generally tried to have our structs be immutable so once this goes in I'm probably going to have a peek at whatever is left. :-)

[eerhardt]

There's a few of them left. The biggest buckets are:

• structs used in C++ interop/field layout scenarios.
  • ex.

    machinelearning/src/Microsoft.ML.Data/Model/ModelHeader.cs

    Line 15 in 14dadfe

    public struct ModelHeader

  • machinelearning/src/Microsoft.ML.TensorFlow/TensorFlow/Tensorflow.cs

    Line 1835 in 65c3c7c

    internal struct TFInput

• Evaluator/metric type structs:
  • These can probably be easily fixed.
  • ex.

    machinelearning/src/Microsoft.ML.Data/Evaluators/AucAggregator.cs

    Line 215 in 9d583b6

    public struct AucInfo

  • machinelearning/src/Microsoft.ML.Data/Evaluators/EvaluatorUtils.cs

    Line 21 in a8bc268

    public struct AggregatedMetric

And there are other cases, but more on a 'case-by-case' basis.