Parse less JSON on null builds

This commit fixes a performance pathology in Cargo today. Whenever Cargo
generates a lock file (which happens on all invocations of `cargo build`
for example) Cargo will parse the crates.io index to learn about
dependencies. Currently, however, when it parses a crate it parses the
JSON blob for every single version of the crate. With a lock file,
however, or with incremental builds only one of these lines of JSON is
relevant. Measured today Cargo building Cargo parses 3700 JSON
dependencies in the registry.

This commit implements an optimization that brings down the number of
parsed JSON lines in the registry to precisely the right number
necessary to build a project. For example Cargo has 150 crates in its
lock file, so now it only parses 150 JSON lines (a 20x reduction from
3700). This in turn can greatly improve Cargo's null build time. Cargo
building Cargo dropped from 120ms to 60ms on a Linux machine and 400ms
to 200ms on a Mac.

The commit internally has a lot more details about how this is done but
the general idea is to have a cache which is optimized for Cargo to read
which is maintained automatically by Cargo.

Closes rust-lang#6866

Cargo.toml

@@ -44,6 +44,7 @@ lazycell = "1.2.0"
 libc = "0.2"
 log = "0.4.6"
 libgit2-sys = "0.7.9"
+memchr = "2.1.3"
 num_cpus = "1.0"
 opener = "0.3.0"
 rustfix = "0.4.4"

src/cargo/core/mod.rs

@@ -14,6 +14,7 @@ pub use self::shell::{Shell, Verbosity};
 pub use self::source::{GitReference, Source, SourceId, SourceMap};
 pub use self::summary::{FeatureMap, FeatureValue, Summary};
 pub use self::workspace::{Members, Workspace, WorkspaceConfig, WorkspaceRootConfig};
+pub use self::interning::InternedString;
 
 pub mod compiler;
 pub mod dependency;