Add-Type ReadAllText perf improvement

Add-Type uses File.ReadAllText() API in a loop for reading in multiple source files. This can be non-performant for multiple reasons: a. ReadAllText() uses a small buffer that increases IO calls. b. It performs StringBuilder.ToString() internally which allocates a string that is not used, making possible LOH allocation and adding GC pressure.

Consider using a different pattern:

StringBuilder sourceCode = new StringBuilder();

foreach (string file in paths)
{
    foreach (string line in File.ReadAllLines(file))
    {
        sourceCode.AppendLine(line);
    }
}

String result = sourceCode.ToString();

[lzybkr]

I don’t think we’ve typically viewed the public api surface of a cmdlet implementation as a real C# api, so I’m fine with changes like this.

There might be a rare exception, and if we find out we made a mistake, it’s generally much easier to fix that these days, as compared to waiting multiple years between releases.

[SteveL-MSFT]

@PowerShell/powershell-committee reviewed this and is ok with the changes already made to remove AddTypeCommandBase class