Removed need for -Z build-std

[catt-io]

Fixes the error discussed at pd-rs/crankstart#30 by removing the need to build std from source. That does appear to be a bug caused from a regression in rust nightly, but this may still be a better option to lower the reliance on nightly rust.

Nightly is still necessary for the aloc feature, update readme to accommodate.

[lilyinstarlight]

Is it possible to make this configurable?

[rtsuk]

What is the benefit of making this configurable?

[lilyinstarlight]

What is the benefit of making this configurable?

I don't use rustup and build-std works fine for me (and propagating an appropriate rust-std build without rustup may be annoying). But if allowing build-std (or even just making cargo flags in general configurable via environment variable, e.g. CRANK_CARGO_FLAGS) is prohibitively difficult, I can probably figure out how to propagate a thumbv7em-none-eabihf build of rust-std separately

So if it's too difficult, it doesn't have to hold this up

[rtsuk]

Makes sense. If you want to add in a flag for it, I'll merge it.

[boozook]

-Z build-std needed to build alloc and use heaped c-strings (CString) properly. Strings is just as example. We're need to build alloc crate for this target.

[boozook]

I've return build-std just now in the #42, but in nearest future I'll try to get it configurable as requested.