Updated default Rust version to 1.49.0 to fix bazel sync

[UebelAndre]

Having the aarch64-apple-darwin target in the list of default toolchains breaks bazel sync with the following error:

INFO: Repository rust_darwin_aarch64 instantiated at:
  /Users/user/Code/cargo-raze/WORKSPACE.bazel:19:18: in <toplevel>
  /private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/io_bazel_rules_rust/rust/repositories.bzl:110:24: in rust_repositories
  /private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/io_bazel_rules_rust/rust/repositories.bzl:714:30: in rust_repository_set
Repository rule rust_toolchain_repository defined at:
  /private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/io_bazel_rules_rust/rust/repositories.bzl:613:44: in <toplevel>
WARNING: Download from https://static.rust-lang.org/dist/rust-1.48.0-aarch64-apple-darwin.tar.gz failed: class com.google.devtools.build.lib.bazel.repository.downloader.UnrecoverableHttpException GET returned 404 Not Found
ERROR: An error occurred during the fetch of repository 'rust_darwin_aarch64':
   Traceback (most recent call last):
	File "/private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/io_bazel_rules_rust/rust/repositories.bzl", line 578, column 44, in _rust_toolchain_repository_impl
		build_components = [_load_rust_compiler(ctx)]
	File "/private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/io_bazel_rules_rust/rust/repositories.bzl", line 483, column 24, in _load_rust_compiler
		load_arbitrary_tool(
	File "/private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/io_bazel_rules_rust/rust/repositories.bzl", line 442, column 17, in load_arbitrary_tool
		ctx.download(
Error in download: java.io.IOException: Error downloading [https://static.rust-lang.org/dist/rust-1.48.0-aarch64-apple-darwin.tar.gz] to /private/var/tmp/_bazel_user/32618c3132899251ff30687e197d6243/external/rust_darwin_aarch64/rust-1.48.0-aarch64-apple-darwin.tar.gz: GET returned 404 Not Found

With the release of Rust version 1.49.0 (and the tooling for aarch64-apple-darwin), bazel sync once again succeeds.
Closes #506

Additionally, a Bazel target has been created for //util:fetch_shas.sh (//util:fetch_shas) and made compatible with MacOS which was used to update the sha256 map.

Users on MacOS will need to install GNU parallel which can be done via

brew install parallel

[UebelAndre]

https://blog.rust-lang.org/inside-rust/2020/12/29/1.49.0-prerelease.html might actually change the nature of this pull request. If a compiler is released for aarch64-apple-darwin then instead the default version should change to 1.49 and the appropriate sha256s should be added. I still think this is worth while to merge since it fixes currently broken functionality.

[UebelAndre]

Ok, now that there is a compiler for aarch64-apple-darwin, this PR has changed to update the default version instead of removing the declaration of the toolchain.

[UebelAndre]

With 1.49.0 I ran into a new clippy error:

INFO: From Clippy test/clippy/ok_test_clippy.ok:
error: identical args used in this `assert_eq!` macro call
 --> test/clippy/src/lib.rs:9:20
  |
9 |         assert_eq!(2 + 2, 4);
  |                    ^^^^^^^^
  |
  = note: `#[deny(clippy::eq_op)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#eq_op

error: aborting due to previous error

A fix for this has also been added to this PR.

[illicitonion]

Thanks! One small nit and I think we're good to go