Move build/howl root anywhere

Currently:
- howl root under `Assets/`
- build root under `Assets/~build` (may be moved/renamed safely, anywhere in the `Assets` directory, provided Unity is closed while doing so)

**Pro** - putting derived products in a `~build` dir is intuitive; mixing Howl and C# in the same project remains intuitive.
**Con** - imported C# sources are moved. In *general* not a problem, yet some packages/tools may not like this. Also, until own UI for asmdef is available, managing that feels clunky.

Initially, was doing it the other way (C# under `Assets/`, Howl under `Assets/[ProjectName].Howl/`). Not intuitive; however, does not require moving files.

*Anywhere* would imply side by side, one inside another or perhaps *outside* Unity projects, or in *packages*. Likely dev/deployment scenarios for any of these.