Replace StringBuilder with ValueStringBuilder in AssemblyName.FullName

[stephentoub]

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
using System.Globalization;
using System.Linq;
using System.Reflection;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private AssemblyName[] _names = AppDomain.CurrentDomain.GetAssemblies().Select(a => new AssemblyName(a.FullName)).ToArray();

    [Benchmark]
    public int Names()
    {
        int sum = 0;
        foreach (AssemblyName name in _names)
        {
            sum += name.FullName.Length;
        }
        return sum;
    }
}

Method	Toolchain	Mean	Error	StdDev	Ratio	Allocated
Names	\main\corerun.exe	3.654 us	0.0285 us	0.0238 us	1.00	10 KB
Names	\pr\corerun.exe	2.205 us	0.0202 us	0.0189 us	0.60	2 KB

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

Thanks!