Drop use of hashbrown

Since Rust 1.36, this is the default HashMap implementation so there is
no need to pull in an external crate.

This essentially reverts 636412f.

Cargo.toml

@@ -23,7 +23,6 @@ env_logger = "0.5"
 failure = "~0.1.2"
 fuse = "0.3"
 getopts = "0.2"
-hashbrown = "0.1"
 log = "0.4"
 nix = "0.12"
 serde = "1.0"

NEWS.md

@@ -4,10 +4,6 @@
 
 **STILL UNDER DEVELOPMENT; NOT RELEASED YET.**
 
-* Switched to the hashbrown implementation of Swiss Tables for hash maps,
-  which brings an up to 1% performance improvement during Bazel builds
-  that use sandboxfs.
-
 * Fixed the definition of `--input` and `--output` to require an argument,
   which makes `--foo bar` and `--foo=bar` equivalent.  This can be thought to
   break backwards compatibility but, in reality, it does not.  The previous

src/lib.rs

@@ -31,7 +31,6 @@
 #[cfg(feature = "profiling")] extern crate cpuprofiler;
 #[macro_use] extern crate failure;
 extern crate fuse;
-extern crate hashbrown;
 #[macro_use] extern crate log;
 extern crate nix;
 extern crate serde_derive;
@@ -41,9 +40,9 @@ extern crate signal_hook;
 extern crate time;
 
 use failure::{Fallible, Error, ResultExt};
-use hashbrown::HashMap;
 use nix::errno::Errno;
 use nix::{sys, unistd};
+use std::collections::HashMap;
 use std::ffi::OsStr;
 use std::fmt;
 use std::fs;

src/nodes/dir.rs

@@ -17,10 +17,10 @@ extern crate time;
 
 use {Cache, create_as, IdGenerator};
 use failure::{Fallible, ResultExt};
-use hashbrown::HashMap;
 use nix::{errno, fcntl, sys, unistd};
 use nix::dir as rawdir;
 use nodes::{ArcHandle, ArcNode, AttrDelta, Handle, KernelError, Node, NodeResult, conv, setattr};
+use std::collections::HashMap;
 use std::ffi::{OsStr, OsString};
 use std::os::unix::ffi::OsStrExt;
 use std::os::unix::fs::{self as unix_fs, DirBuilderExt, OpenOptionsExt};