basis for a resident compiler

unstable, but can be tested with -Dsbt.resident.limit=n
 n is the maximum Globals kept around

interface/src/main/java/xsbti/compile/GlobalsCache.java

@@ -0,0 +1,10 @@
+package xsbti.compile;
+
+import xsbti.Logger;
+import xsbti.Reporter;
+
+public interface GlobalsCache
+{
+	public CachedCompiler apply(String[] args, boolean forceNew, CachedCompilerProvider provider, Logger log, Reporter reporter);
+	public void clear();
+}