sdk: Extract solana-sysvar crate

[joncinque]

Problem

The sysvars in solana-program are tightly coupled with types that exist in solana-program. For example, all of the special sysvar getters like Rent::get() are implemented through a macro that falls back to using program_stubs.

Because of this tight coupling, it's difficult to pull out bits for the sysvars.

Summary of changes

After numerous attempts, I've decided to keep it simple and only extract SysvarId, its helper macros, and get_sysvar.

To go along with that, all of the separated sysvar crates now implement the sysvar ids themselves under a new sysvar feature. This new feature might be overkill, so let me know if we should just include the sysvar ids by default. I went with a feature to include an implementation using the sol_get_sysvar syscall in the future.

It was really messy to include the Sysvar trait from solana-program because it falls back to using bincode, which we know performs poorly for on-chain programs. So the future idea is to create a new Sysvar trait in solana-sysvar which will require fewer bits to deserialize sysvars.

Let me know what you think about this PR and the future idea! Note that I'll need to rebase this on top of #3249 and #3272 when they land.

[febo]

Maybe I missed something, but my current understanding is that we would still need to use both the sysvar specific crate + solana-program to get a sysvar. This is because impl_sysvar_get! still under solana-program.