Backend nearly entirely converted to D

[Walter Bright]

With the recent merging of the last of the big files machobj.d:

https://github.com/dlang/dmd/pull/8911

I'm happy to say we're over the hump in converting the backend to D!

Remaining files are minor: tk.c, cgen.c, dt.c, fp.c, os.c, outbuf.c, 
sizecheck.c, strtold.c and mem.c. I'll probably leave a couple in C anyway - 
os.c and strtold.c. sizecheck.c will just go away upon completion.

Thanks to everyone who helped out with this!

Of course, the code remains as ugly as it was in C. It'll take time to bit by 
bit refactor it into idiomatic D.

The more immediate benefit is to get rid of all the parallel .h files, which 
were a constant source of bugs when they didn't match the .d versions.