Support for stable Rust

[oleganza]

This adds support for using stable Rust by introducing a new cargo feature nightly. Since all the docs are using unstable external_doc compiler feature, the docs can only be build on a nightly Rust.

# basic features on stable:
cargo build

# with R1CS API on stable:
cargo build --features="yoloproofs"

# no_std (nightly only):
cargo build --no-default-features --features="nightly"

# vectorized backend (nightly only):
cargo build --no-default-features --features="avx2_backend"

# build documentation (nightly only)
make doc-internal

Since Travis CI is not reliable anymore, we also move to Github Actions. Both main and develop branches are checked against stable and nightly Rust. Main branch does not use yoloproofs feature.

For development, we keep rust-toolchain with the latest available nightly. If it ever breaks, one can switch over to stable instead of hunting down a more workable version of nightly (although, docs won't build).

(Replaces #314)

[hdevalence]

Didn't test, but this looks great to me.

[oleganza]

i think that's the end for Travis