Ported js13k game underrun to D targeting webassembly

[Sebastiaan Koppe]

Underrun is small game build by Dominic Szablewski for the 2018 
js13kGames competition.

I decided to port it to D and to target webassembly. You can play 
the game here https://skoppe.github.io/spasm/examples/underrun/

It is part of my endeavour into the wonderful world of 
webassembly. I specifically choose this project so I could get a 
feel for how to call web api's other than the dom, i.e. WebGL and 
AudioContext.

Porting went pretty smooth, except for the fact that I wanted it 
to be done in betterC, which turned out to cripple lots of things 
I am used to rely on.

Things like:

- having to write closures by hand
- not being able to use stdx.allocator
- almost no use of phobos (even at CTFE)

D could definitely use some love in that area. I was specifically 
stumped that stdx.allocator doesn't work in betterC; if somewhere 
is a good place to use stdx.allocator you would expect it to be 
betterC, but alas.

Had lots of fun though. Hope you like it.