Reduce allocations of Record/Array readers

[gmanvel]

This PR reduces allocations for Record & Array readers. While profiling memory footprint of deserialization

Fixture fixture = new Fixture();
var data = fixture
    .Build<User>()
    .With(u => u.Offerings, fixture.CreateMany<Offering>(50).ToList)
    .CreateMany(1000)
    .ToArray();

var serialized = AvroConvert.Serialize(data);

Console.WriteLine($"Serialized {serialized.Length}");
Console.ReadLine();

var deserialized = AvroConvert.Deserialize<User[]>(serialized);

Console.WriteLine($"Deserialized {deserialized.Length} users");

noticed few allocations that can be removed

In particular closure & ReadOnlyCollection<string> allocations from ResolveRecord

Avoid resizing list by setting its initial capacity on creation

As a result memory footprint improved

and ~ 9% improvement in Deserialize execution

Method	Mean	Error	StdDev	Allocated
Deserialize	62.26 ms	1.244 ms	1.574 ms	30.76 MB

VS

Method	Mean	Error	StdDev	Allocated
Deserialize	56.59 ms	1.108 ms	1.820 ms	27.34 MB

For reference, benchmark code

[MemoryDiagnoser(displayGenColumns: false)]
[HideColumns("RatioSD")]
public class ReaderBenchmarks
{
    private byte[] _serializedUsers;

    [GlobalSetup]
    public void Setup()
    {
        var fixture = new Fixture();
        var data = fixture
            .Build<User>()
            .With(u => u.Offerings, fixture.CreateMany<Offering>(50).ToList)
            .CreateMany(1000)
            .ToArray();

        _serializedUsers = AvroConvert.Serialize(data);
    }

    [Benchmark]
    public int Deserialize() => AvroConvert.Deserialize<User[]>(_serializedUsers).Length;
}