Make all heap objects store a reference to their class and inline class lookup.

This is redundant, but significantly optimizes method dispatch:

binary_trees - wren           ..........  3347  0.30s  110.30% relative to baseline
delta_blue - wren             ..........  7161  0.14s  110.39% relative to baseline
fib - wren                    ..........  2445  0.41s  112.78% relative to baseline
for - wren                    ..........  8600  0.12s  143.00% relative to baseline
method_call - wren            ..........  5112  0.20s  114.15% relative to baseline

Makefile

@@ -17,10 +17,10 @@ RELEASE_OBJECTS = $(addprefix build/release/, $(notdir $(OBJECTS)))
 all: release
 
 clean:
-	rm -rf build wren wrend
+	@rm -rf build wren wrend
 
 prep:
-	mkdir -p build/debug build/release
+	@mkdir -p build/debug build/release
 
 # Debug build.
 debug: prep wrend