all: store data layout as little endian value

This makes it much easier to read the value at runtime, as pointer
indices are naturally little endian. It should not affect anything else
in the program.

interp/memory.go

@@ -1289,6 +1289,7 @@ func (r *runner) readObjectLayout(layoutValue value) (uint64, *big.Int) {
 		}
 		rawBytes[i] = byte(v)
 	}
+	reverseBytes(rawBytes) // little-endian to big-endian
 	bitmap := new(big.Int).SetBytes(rawBytes)
 	return objectSizeWords, bitmap
 }
@@ -1338,3 +1339,12 @@ func (r *runner) getLLVMTypeFromLayout(layoutValue value) llvm.Type {
 	}
 	return llvmLayoutType
 }
+
+// Reverse a slice of bytes. From the wiki:
+// https://github.com/golang/go/wiki/SliceTricks#reversing
+func reverseBytes(buf []byte) {
+	for i := len(buf)/2 - 1; i >= 0; i-- {
+		opp := len(buf) - 1 - i
+		buf[i], buf[opp] = buf[opp], buf[i]
+	}
+}