Improve performace of repeated packed fixedSize fields

[YarinOmesi]

Changes

Improve performance of packed RepeatedField that has fixed size, by coping LEN bytes from serialzied buffer to RepeatedField array (like c memcpy).

Relevant Information

As the docs here notes, fixed size value is memcpy of the equivalent C types (u?int64_t, double)

You can see benchmark code and results here

Summarized result:

Method	Job	Size	Mean	Ratio	Allocated
Parse_RepeatedFloats	3.29.0	100000	416,582.7 ns	baseline	400297 B
Parse_RepeatedFloats	improved-fixed-size-repeated	100000	134,671.7 ns	-68%	400776 B
Write_RepeatedFloats	3.29.0	100000	340.549 us	baseline	391.57 KB
Write_RepeatedFloats	improved-fixed-size-repeated	100000	106.577 us	-69%	391.5 KB

---

I am pretty new to this repo to tell me what to you think about this.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[JasonLunn]

@YarinOmesi - can you rebase this PR to see if that clears up the test failures?

[JasonLunn]

Can you take a look at the failing C# conformance test?

Unhandled exception. System.InvalidOperationException: Type registry has no descriptor for type name ''
   at Google.Protobuf.JsonParser.MergeAny(IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.<>c.<.cctor>b__42_6(JsonParser parser, IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.Merge(IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.TryParseSingleValue(FieldDescriptor field, JsonTokenizer tokenizer, Object& value)
   at Google.Protobuf.JsonParser.MergeField(IMessage message, FieldDescriptor field, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.Merge(IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.Merge(IMessage message, TextReader jsonReader)
   at Google.Protobuf.JsonParser.Parse[T](TextReader jsonReader)
   at Google.Protobuf.JsonParser.Parse[T](String json)

[YarinOmesi]

Can you take a look at the failing C# conformance test?

Unhandled exception. System.InvalidOperationException: Type registry has no descriptor for type name ''
   at Google.Protobuf.JsonParser.MergeAny(IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.<>c.<.cctor>b__42_6(JsonParser parser, IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.Merge(IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.TryParseSingleValue(FieldDescriptor field, JsonTokenizer tokenizer, Object& value)
   at Google.Protobuf.JsonParser.MergeField(IMessage message, FieldDescriptor field, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.Merge(IMessage message, JsonTokenizer tokenizer)
   at Google.Protobuf.JsonParser.Merge(IMessage message, TextReader jsonReader)
   at Google.Protobuf.JsonParser.Parse[T](TextReader jsonReader)
   at Google.Protobuf.JsonParser.Parse[T](String json)

There was a problem with reading bools with my approch because of .net marshaling stuff, so i added validation that comapre .net size and codec.FixedSize