(Request) Anchor Bindings and a token-lending-common crate

[0xripleys]

1. kind of annoying if users have to depend on the entire solend program if they want to use our structs. there's a non-zero change for dependency conflict.
2. some people prefer using anchor bindings to make cpi calls.

I had a PR for this but it went stale #98 not too much work to revive it i guess