Initial implementation of project-lock.json.

This commit adds a initial implementation of project-lock.json, a build
system agnostic method of specifying the crate graph and roots.

crates/ra_project_model/src/json_project.rs

@@ -0,0 +1,49 @@
+use std::path::PathBuf;
+
+use serde::Deserialize;
+
+/// A root points to the directory which contains Rust crates. rust-analyzer watches all files in
+/// all roots. Roots might be nested.
+#[derive(Clone, Debug, Deserialize)]
+#[serde(transparent)]
+pub struct Root {
+    pub(crate) path: PathBuf,
+}
+
+/// A crate points to the root module of a crate and lists the dependencies of the crate. This is
+/// useful in creating the crate graph.
+#[derive(Clone, Debug, Deserialize)]
+pub struct Crate {
+    pub(crate) root_module: PathBuf,
+    pub(crate) edition: Edition,
+    pub(crate) deps: Vec<Dep>,
+}
+
+#[derive(Clone, Copy, Debug, Deserialize)]
+#[serde(rename = "edition")]
+pub enum Edition {
+    #[serde(rename = "2015")]
+    Edition2015,
+    #[serde(rename = "2018")]
+    Edition2018,
+}
+
+/// Identifies a crate by position in the crates array.
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[serde(transparent)]
+pub struct CrateId(pub usize);
+
+/// A dependency of a crate, identified by its id in the crates array and name.
+#[derive(Clone, Debug, Deserialize)]
+pub struct Dep {
+    #[serde(rename = "crate")]
+    pub(crate) krate: CrateId,
+    pub(crate) name: String,
+}
+
+/// Roots and crates that compose this Rust project.
+#[derive(Clone, Debug, Deserialize)]
+pub struct JsonProject {
+    pub(crate) roots: Vec<Root>,
+    pub(crate) crates: Vec<Crate>,
+}