Split robot collision checking into separate compilation units

**Is your feature request related to a problem? Please describe.**
Compilation times are growing, and they scale linearly with the number of supported robots. It gets even worse with complex robots like Spot (#67).

**Describe the solution you'd like**
We can resolve this by splitting robots' code into separate compilation units, compiling them in parallel, and then linking it all together.

**Describe alternatives you've considered**
Maybe just get things to compile faster? Or just live with the pain.

**Additional context**
We should probably also allow optionally-enabled robots in compilation as well.