pallet-contracts: Rent projection RPC (#4754)

* Initial approach

* Introduce the pallet-contracts-common crate

* Add rent::compute_rent_projection

* Wire everything together

* Fix build error.

* Rename EvictionDate → EvictionAt.

* Clean.

* Renaming and cleaning.

* Add documentation for rent_projection RPC.

* Add documentation for rent_projection runtime API.

* Refactor rent_budget.

Merge it with subsistence_treshold.

* Bump impl_version

* Constrain RPC impl with Block::Header::Number.

* Rename pallet-contracts-common into -primitives

* Add a comment for `compute_rent_projection` on the usage

* Small tidying

substrate/frame/contracts/common/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "pallet-contracts-primitives"
+version = "2.0.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+[dependencies]
+# This crate should not rely on any of the frame primitives.
+codec = { package = "parity-scale-codec", version = "1.0.0", default-features = false, features = ["derive"] }
+sp-std = { version = "2.0.0", default-features = false, path = "../../../primitives/std" }
+sp-runtime = { version = "2.0.0", default-features = false, path = "../../../primitives/runtime" }
+
+[features]
+default = ["std"]
+std = [
+	"codec/std",
+	"sp-runtime/std",
+	"sp-std/std",
+]