This PR improves compatibility with RISC-V and PolkaVM, allowing more
runtimes to successfully compile.

In particular, it makes the following changes:

- The `sp-mmr-primitives` and `sp-consensus-beefy` crates
unconditionally required an `std`-only dependency; now they only require
those dependencies when the `std` feature is actually enabled. (Our
RISC-V target is, unlike WASM, a true `no_std` target where you can't
accidentally use stuff from `std` anymore.)
- One of our dependencies (the `bitvec` trace) uses a crate called
`radium` which doesn't compile under RISC-V due to incomplete
autodetection logic in their `build.rs` file. The good news is that this
is already fixed in the newest upstream version of `radium`, and the
newest version of `bitvec` uses it. The bad news is that the newest
version of `bitvec` is not currently released on crates.io, so we can't
use it. I've [created an
issue](https://github.com/ferrilab/ferrilab/issues/5) asking for a new
release, but in the meantime I forked the currently used `radium` 0.7,
[fixed the faulty
logic](https://github.com/paritytech/radium-0.7-fork/commit/ed66c8a294b138c67f93499644051d97d4c7fbda)
and used cargo's patching capabilities to use it for the RISC-V runtime
builds. This might be a little hacky, but it is the least intrusive way
to fix the problem, doesn't affect WASM builds at all, and we can
trivially remove it once a new `bitvec` is released.
- The new runtimes are added to the CI to make sure their compilation
doesn't break.