Built in function to count the number of bits set

Suppose you have a `bits` type like the following:

```
bits Features:
0 [+1] Flag foo
$next [+1] Flag bar
$next [+1] Flag baz
$next [+1] Flag abc
$next [+1] Flag xyz
```

A packet contains a bitset like the one above with the number of flags enabled determining the size of a variable length array.

```
struct Data:
0 [+1] Features features

let num_features = (features.foo ? 1 : 0) + (features.bar ? 1 : 0) + ...
$next [+num_features] UInt feature_values
```

The calculation of `num_features` can become quite a bit of toil. We have to physically write out the bits we wish to check. It would be easy to forget to add here if another needed to be checked in the future. It would be much easier to simply call out to a built in function which can do the counting for us.

Something like this would be so much better:

```
struct Data:
0 [+1] Features features

let num_features = bits_set(features)
$next [+num_features] UInt feature_values
```