No constants

```
var opcodes = constants.opcodes;
^

TypeError: Cannot read property 'opcodes' of undefined
```

bcoin.constants no more?

Start with the code containing `var opcodes = constants.opcodes` and trace how `constants` is obtained. Reproduce the TypeError and compare that usage with the bcoin API available to this project; done means the constants lookup works or the supported incompatibility is clearly documented.

Written by the indexing model from the issue text.