Change `CodeDomEvaluator.IsCachingEnabled` default to `true`

[oleg-shilo]

Triggered by #433

"That's why caching (in hosted scenarios) is disabled by default."
Now caching has a very different impact on Roslyn vs. CodeDom. With Roslyn the benefits are small, but with CodeDom they are huge. Would changing the default to "enabled by default" for CodeDom be an option? Or only point at this difference in the wiki?

[maettu-this]

Let me add a caching related question/issue. Feel free to extract into a separate issue if you think it doesn't belong here.

Runtime.cs restricts "does not support the cache directory for class library projects", resulting in a host application not being able to retrieve CacheDir. While it can replicate CacheDir = GetScriptTempDir().PathJoin("cache"), this obviously is something different than GetCacheDir(). I am not really getting this piece of code.

Or stated differently, could Runtime provide...

public static string GetCacheDir() => CacheDir;

...on #if class_lib?

Or is CSExecutor.GetCacheDirectory() something that could be made public?

Note the differences among naming of SetScriptCacheDir() vs. GetCacheDirectory(). Should probably be better aligned.

Also, note that with my host app CS-Script creates two differently named directories in "\Temp":

"C:\Users<USER>\AppData\Local\Temp\CSSCRIPT\cache" which contains build info of the scripts.
"C:\Users<USER>\AppData\Local\Temp\csscript.core" which remains empty.

Background: My host app outputs verbose information on compiling and loading a script, just like CS-Script -verbose does. But currently there is no method to retrieve the cache dir.

[oleg-shilo]

I extracted your comment into a separate issue: #446