vkd3d-shader/d3d-asm: Dump the input and output signatures.

At some point I would like to have an assembler for TPF, so that it's easier to write and modify tests. This is a first step in that direction, fixing some kind of format for serializing signatures in the comment at the beginning of the assembler code. I'm not decided yet on all details, so take this as an RFC for the moment.