Find the library hash used by a toolchain

[brson]

For custom linking purposes, sometimes people need to know the hash used in std's filename. It was suggested that show hash could be used for this.

It's a real use case, but I'm not sure yet that it's rustup's job, or if it is that it shouldn't be folded into some other version-displaying function.

[m4b]

For reference, I'm working on a Makefile for hopefully doing the linking in a more extensible manner, and I currently grab the hash with a huge hack, e.g.:

PREFIX=$(HOME)/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu
LIB=$(PREFIX)/lib
RUSTLIB=$(LIB)/rustlib/x86_64-unknown-linux-musl/lib
HASH=$(shell ls $(RUSTLIB) | grep libstd | grep -oe "-[[:alnum:]]*" | grep -oe "[[:alnum:]]*")

(also, and if anyone else looks at the Makefile, what I'm doing should not be understood at all as a typical Rust build process)

There's so many places the above can break tho, e.g., if multirust is installed somewhere different, if the rustlib nightly dir changes (although you mentioned should be stablized soon), etc.

I agree not sure if it's rustup's job, or what all that would look like, but yea, hopefully a good solution arises somewhere :)