Install creusot-rustc in a toolchain-dependent location

[Lysxia]

Fixes #671.

Problem: creusot-rustc is dynamically linked with rustc_driver, so it can't be run
directly from the command line because it can't find its dependencies alone.

Solution: Putting it in a special directory controlled by Creusot:

1. removes it from $PATH, suppressing the temptation to run it from the command line
2. allows cargo-creusot to detect incomplete upgrades (this prevents
   an obscure dynlink error if one updates the toolchain that creusot depends
   on and only reinstalls one of cargo-creusot or creusot-rustc).

The installation of creusot-rustc is now handled by cargo creusot setup install.

[arnaudgolfouse]

So I see that the rust toolchain version is now fully built-in inside cargo-creusot, does that mean that creusot will work even if the user does not explicitly set a version with a rust-toolchain file? I'm on the fence on whether this is desirable or not, since this mean you could do the following:

• build with creusot, everything works
• try to build with rustc, things break because of nightly features

Also I can see this confusing rust-analyzer, although this is probably ok now that creusot has its own target directory.

[Lysxia]

I was hoping that things would "just work" in simple cases, but now that you mention it, we can't even build creusot-contracts on a stable toolchain. I'm going to open an issue about it.