Remove the `compiler-builtins` feature from default because it prevents
testing via the default `cargo test` command. It made more sense as a
default when `compiler-builtins` was a dependency that some crates added
via crates.io, but is no longer needed.
The `rustc-dep-of-std` feature is also removed since it doesn't do
anything beyond what the `compiler-builtins` feature already does.
Trevor Gross
59fe28d0ff