Revert use of collectable assemblies for eval

It turns out they've only been supported in Mono since Feb 09 2010.
This does mean eval will leak memory.  Try not to overuse it.

Makefile

@@ -2,7 +2,7 @@
 
 # How to run CLR programs; can be blank for Win32
 RUN_CLR=mono
-CSC=dmcs
+CSC=gmcs
 RM=rm -f
 CP=cp
 

README.pod

@@ -15,9 +15,17 @@ C<test.pl>.
 
 =item *
 
-To run on Mono: You will need 2.8.x or higher.  2.8.2 or later is
-recommended for best performance; you should use C<RUN_CLR=mono-sgen>
-in this case.  2.10 (latest stable) is also tested.
+To run on Mono:  2.6.4 and later are well-tested, though success has been
+reported with 2.4.x as well.  ggoebel on #perl6 reports that 2.6.7 fails
+with a bus error on OS X 10.4 x86, but 2.6.4 works; 2.6.7 is fine on other
+platforms.
+
+2.8.2 or later is recommended for best performance; you should use
+C<RUN_CLR=mono-sgen> in this case.  2.10 (latest stable) is also tested.
+
+Debian and derivatives have a modular mono packaging; you should install
+'mono-complete'.  (The other packages exist for the sake of Debian-packaged
+programs.)
 
 To run on Windows/Mono (2.8.2 and 2.10) and other systems, download the
 mono installers from C<http://go-mono.org/>

lib/CLRBackend.cs

@@ -3558,7 +3558,7 @@ public class CLRBackend {
             AssemblyName an = new AssemblyName(mobname);
             this.dir = dir;
             ab = AppDomain.CurrentDomain.DefineDynamicAssembly(an,
-                    (filename == null ? AssemblyBuilderAccess.RunAndCollect :
+                    (filename == null ? AssemblyBuilderAccess.Run :
                         AssemblyBuilderAccess.Save), dir);
             mob = filename == null ? ab.DefineDynamicModule(mobname) :
                 ab.DefineDynamicModule(mobname, filename);