Use crossbeam_utils instead crossbeam

[l4l]

It seems that crossbeam only used for threading, so no need to depending on the whole framework, but only utils part.

Before:

fuse-rust v0.1.3
└── crossbeam v0.7.3
    ├── cfg-if v0.1.10
    ├── crossbeam-channel v0.4.4
    │   ├── crossbeam-utils v0.7.2
    │   │   ├── cfg-if v0.1.10
    │   │   └── lazy_static v1.4.0
    │   │   [build-dependencies]
    │   │   └── autocfg v1.0.1
    │   └── maybe-uninit v2.0.0
    ├── crossbeam-deque v0.7.3
    │   ├── crossbeam-epoch v0.8.2
    │   │   ├── cfg-if v0.1.10
    │   │   ├── crossbeam-utils v0.7.2 (*)
    │   │   ├── lazy_static v1.4.0
    │   │   ├── maybe-uninit v2.0.0
    │   │   ├── memoffset v0.5.6
    │   │   │   [build-dependencies]
    │   │   │   └── autocfg v1.0.1
    │   │   └── scopeguard v1.1.0
    │   │   [build-dependencies]
    │   │   └── autocfg v1.0.1
    │   ├── crossbeam-utils v0.7.2 (*)
    │   └── maybe-uninit v2.0.0
    ├── crossbeam-epoch v0.8.2 (*)
    ├── crossbeam-queue v0.2.3
    │   ├── cfg-if v0.1.10
    │   ├── crossbeam-utils v0.7.2 (*)
    │   └── maybe-uninit v2.0.0
    └── crossbeam-utils v0.7.2 (*)

and after:

fuse-rust v0.1.3
└── crossbeam-utils v0.8.1
    ├── cfg-if v1.0.0
    └── lazy_static v1.4.0
    [build-dependencies]
    └── autocfg v1.0.1

[Blakeinstein]

Brilliant!
I was already very skeptical of including a library to have scoped threads... with my goal being a no dependency library. Thanks for this! I will push this to release as well, I guess.