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_leaking.go

@@ -0,0 +1,47 @@
+// +build gc.leaking
+
+package runtime
+
+// This GC implementation is the simplest useful memory allocator possible: it
+// only allocates memory and never frees it. For some constrained systems, it
+// may be the only memory allocator possible.
+
+import (
+	"unsafe"
+)
+
+// Ever-incrementing pointer: no memory is freed.
+var heapptr = heapStart
+
+func alloc(size uintptr) unsafe.Pointer {
+	// TODO: this can be optimized by not casting between pointers and ints so
+	// much. And by using platform-native data types (e.g. *uint8 for 8-bit
+	// systems).
+	size = align(size)
+	addr := heapptr
+	heapptr += size
+	if heapptr >= heapEnd {
+		runtimePanic("out of memory")
+	}
+	for i := uintptr(0); i < uintptr(size); i += 4 {
+		ptr := (*uint32)(unsafe.Pointer(addr + i))
+		*ptr = 0
+	}
+	return unsafe.Pointer(addr)
+}
+
+func free(ptr unsafe.Pointer) {
+	// Memory is never freed.
+}
+
+func GC() {
+	// No-op.
+}
+
+func KeepAlive(x interface{}) {
+	// Unimplemented. Only required with SetFinalizer().
+}
+
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// Unimplemented.
+}