Use the new feature resolver in Cargo

With the old (default) resolver, you need to pass the `polonius-souffle`
feature to each workspace crate individually when compiling.

Author: ecstatic-morse

Cargo.toml

@@ -8,6 +8,7 @@ repository = "https://github.com/rust-lang-nursery/polonius"
 readme = "README.md"
 keywords = ["compiler", "borrowck", "datalog"]
 edition = "2018"
+resolver = "2"
 
 [dev-dependencies]
 diff            = "0.1.0"

polonius-engine/Cargo.toml

@@ -7,6 +7,7 @@ license = "Apache-2.0/MIT"
 repository = "https://github.com/rust-lang-nursery/polonius"
 readme = "README.md"
 keywords = ["compiler", "borrowck", "datalog"]
+resolver = "2"
 
 [dependencies]
 datafrog = "2.0.0"

polonius-facts/Cargo.toml

@@ -2,6 +2,7 @@
 name = "polonius-facts"
 version = "0.1.0"
 edition = "2018"
+resolver = "2"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 

polonius-souffle/Cargo.toml

@@ -9,6 +9,7 @@ repository = "https://github.com/rust-lang-nursery/polonius"
 readme = "README.md"
 keywords = ["compiler", "borrowck", "datalog"]
 build = "build.rs"
+resolver = "2"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html