Move lazy_static/spin_no_std out of default features and behind a separate feature flag

[str4d]

We currently enable lazy_static/spin_no_std at the request of downstream users who want to use pasta_curves in an alloc no-std environment. However, other downstream users want to not have that flag enabled as they don't want to depend on spin.

rust-lang/cargo#8170 means that we cannot disable a dependency feature flag when we enable one of our own feature flags, so AFAICT the only option to enable both use cases is to add a separate feature flag for enabling lazy_static/spin_no_std, that is off by default (i.e. std by default, matching the rest of the Rust ecosystem). This is a breaking change (as downstream users depending on that will break) so needs to be released in a 0.6.0 release (which we should do anyway for the MSRV bump).