Generate all Fn* internal_fn variant FunApps as CompilerInternal

[seantalts]

Matthijs pointed out in #149 that it's a bit difficult to read the code for generating stuff when you deal with all of the variants in internal_fn in a variety of different places depending on which function it is. We should move to all of these being labeled CompilerInternal, and possibly delete some of them from the variant and just use the StanLib function name where appropriate.

The motivating example was that negative_infinity is actually a StanLib funcction and we'd construct it in the compiler using the StanLib function kind and the FnNegInf variant. But perhaps the thing to actually do is just use the actual string negative_infinity and not create an internal_fn variant for it? OR to call it CompilerInternal and pretend that it doesn't have a StanLib equivalent. I'm not sure. What do you all think? I'm leaning towards one of those two options now - having these special variants sometimes labeled StanLib and sometimes CompilerInternal is not helpful.

[WardBrian]

Done by #856