fix(xsnap): start with expected heap (32MB) and grow by 4MB

flamegraph/perf/zoe.xnap/1814.66/1815.66 shows a lot of time
spent in gc (fxRunID / fxNewChunk / fxMarkValue). This is likely
a result of earlier tweaks to heap parameters (11795de).

We observe that vats usually use around 30MB RSS, so let's start with
that size heap (even though RSS is probably not all XS heap). If we
need more, grow a substantial amount (4MB) at a time.

packages/xsnap/src/xsnap.c

@@ -298,10 +298,10 @@ ExitCode main(int argc, char* argv[])
 		}
 	}
 	xsCreation _creation = {
-		1 * 1024 * 1024,	/* initialChunkSize */
-		512 * 1024,			/* incrementalChunkSize */
-		16 * 1024,			/* initialHeapCount */
-		256 * 1024,			/* incrementalHeapCount */
+		32 * 1024 * 1024,	/* initialChunkSize */
+		4 * 1024 * 1024,	/* incrementalChunkSize */
+		256 * 1024,			/* initialHeapCount */
+		128 * 1024,			/* incrementalHeapCount */
 		4096,				/* stackCount */
 		32000,				/* keyCount */
 		1993,				/* nameModulo */