sort: implement -k and -t support (#1996)

* sort: implement basic -k and -t support

This allows to specify keys after the -k flag and a custom field
separator using -t.

Support for options for specific keys is still missing, and the -b flag
is not passed down correctly.

* sort: implement support for key options

* remove unstable feature use

* don't pipe in input when we expect a failure

* only tokenize when needed, remove a clone()

* improve comments

* fix clippy lints

* re-add test

* buffer writes to stdout

* fix ignore_non_printing

and make the test fail in case it is broken :)

* move attribute to the right position

* add more tests

* add my name to the copyright section

* disallow dead code

* move a comment

* re-add a loc

* use smallvec for a perf improvement in the common case

* add BENCHMARKING.md

* add ignore_case to benchmarks

Cargo.lock

@@ -1,5 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "advapi32-sys"
 version = "0.2.0"
@@ -1362,6 +1364,12 @@ dependencies = [
  "maybe-uninit",
 ]
 
+[[package]]
+name = "smallvec"
+version = "1.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fe0f37c9e8f3c5a4a66ad655a93c74daac4ad00c441533bf5c6e7990bb42604e"
+
 [[package]]
 name = "strsim"
 version = "0.8.0"
@@ -1816,7 +1824,7 @@ dependencies = [
  "quickcheck",
  "rand 0.7.3",
  "rand_chacha",
- "smallvec",
+ "smallvec 0.6.14",
  "uucore",
  "uucore_procs",
 ]
@@ -2289,6 +2297,7 @@ dependencies = [
  "rand 0.7.3",
  "rayon",
  "semver",
+ "smallvec 1.6.1",
  "uucore",
  "uucore_procs",
 ]