all: rename garbage collectors

dumb -> leaking:
  make it more clear what this "GC" does: leak everything.
marksweep -> conservative:
  "marksweep" is too generic, use "conservative" to differentiate
  between future garbage collectors: precise marksweep / mark-compact /
  refcounting.

src/runtime/gc_conservative.go

@@ -1,4 +1,4 @@
-// +build gc.marksweep
+// +build gc.conservative
 
 package runtime
 

src/runtime/gc_leaking.go

@@ -1,4 +1,4 @@
-// +build gc.dumb
+// +build gc.leaking
 
 package runtime