Eagerly resolve toolchains to canonical, fixed reference

[Kha]

This also gets rid of the update-hashes/ directory (it won't literally get rid of it on existing installs but the contents do not matter anymore).

[tydeu]

I would love to see this change! Avoiding multiple installs of the same toolchain under different names would be wonderful (and save me disk space). What will be procedure for users updating elan while already having such toolchains installed (and ones for stable/nightly)?

[Kha]

What will be procedure for users updating elan while already having such toolchain installed (and ones for stable/nightly)?

I would prefer the same simple strategy as getting rid of unused toolchains so far: rm :)