We can just use `lib` instead of `builtins` in all cases but the
`hashString` case. Also changed a few lines to make use of some optional
helpers from lib.

The expression is already long and confusing enough without the color
stuff sprinkled in. Moving it to a dedicated file makes sense.

I switched a bit of the color support code to pure Nix since there
wasn't much point in doing that in bash while we can just do it in Nix.

I already have a few changes in here that will trigger rebuilds so I
might as well do that substitution now.

That code had been in the derivation for a while but no explanation was
given why that is needed. It might be helpful to our future selfs to
document why things are done the way they are.

This should carry the function better then `makeDeps` as it isn't
producing deps but the rustc arguments required to link against those.

This should make it more obvious that we have three parts to it and not
just one long gibberish string that makes up all of it.

We can get rid of a bunch of workarounds that were in the build script
before by just passing on the `crateBin` attribute.

Before we converted the list of attributes to a string only to convert
it back in bash during the build phase. We can do the entire looping
through builds in Nix and thus need no conversion and parsing of
attributes over and over again.

The big part that still remains bash is the heuristic that cargo
introduced and that we can't do at eval time.

The previous lines were only different in the kind of dependencies but
otherwise exactly the same. It makes the entire thing a bit more
readable by moving this into a function that takes care of this.

This means we aren't rebuilding hat file for each crate we are building
and the buildPhase expression is a lot easier to comprehent.

Cleanup buildRustCrate expression