Index and list all available toolchains

[brson]

rustup toolchain list should also tell you which toolchains are available. Getting this information correctly will require either indexing it on the backend or having every client index the archives themselves.

Related to #192

[nodakai]

I wish to have something like --json for as many subcommands as possible. It can be any machine-readable formats, not necessarily JSON

[oconnor663]

Is there a place we can see the available toolchains in the browser right now?

[Diggsey]

@oconnor663 Not in a user-friendly way, but you can find the toml manifests here: https://static.rust-lang.org/dist/index.html

[oconnor663]

@Diggsey thanks, that's what I needed.

[totalknowledge]

rustup toolchain list
gives me a list of the installed toolchains, not the available ones.

[theronic]

Yes, please. The only reason I'll ever run rustup toolchain list is because I'm missing one that I expected to have.

[anowlcalledjosh]

https://static.rust-lang.org/dist/index.html

This appears to not exist any more (the last Internet Archive snapshot was on 11-11-2018) – is there a replacement?

[packysauce]

if you're like me, you probably want rustup target list, because that shows the available target triples, eg x86_64-unknown-linux-musl