Make sure no_std mode works

[bstrie]

Investigation as a part of #54 seems to suggest that ciborium links, if not to libstd, then at least to liballoc, regardless of the presence of the std feature flag. I'm unsure whether or not this is intentional; I assume that disabling the std flag in this crate is intended to disable both liballoc and libstd. I'm unsure when this regression happened (if indeed this is a regression at all), but we should see about fixing it before issuing the next release. We may also want to consider adding an alloc feature.

[chrysn]

Just one data point: I'm using ciborium through the dcaf and coset libraries without pulling in std; I'm building for thumbv7em-none-eabihf which has no std built.

Nonetheless, testing that this property is upheld is quite valuable. Something similar was done in wasmi-labs/wasmi#387, which might be useful as a template.

[ahmedcharles]

The ciborium crate requires alloc, so adding a feature would require changing it to not require alloc. The ciborium-io and ciborium-ll crates already have an alloc feature which seems to work as intended.