normalize install dir structure

Because we initially regarded the CARGO_HOME structure as "overkill" the `~/some/path` and `$MY_ENV_VAR/some/path` installdir strategies do not make a separate `bin/` subfolder, which limits the scalability of the approach somewhat, and adds a ton of special case paths to installers/updaters.

Also the MSI installer really does its own thing altogether when installing to ProgramFiles.

We should change everything to be shaped like CARGO_HOME, with `./bin/` and `./env` (on unix), with room to expand to having libs dirs as well.

This will require migrating existing users of `~/some/path`, but will be good longterm.