deps: drop bytecount in favor of memchr_iter(..).count()

As of the memchr 2.6 release, its Iterator::count method is specialized
to only count the number of occurrences instead of finding the offset of
each occurrence. This replaces ripgrep's use of the bytecount crate.
While micro-benchmarks suggest that memchr's method has better
throughput than bytecount, it turned out to be an illusion. Namely, on a
~13GB haystack prior to this change:

    $ time rg-bytecount 'You killed my friend, my best friend, my lifelong friend!' OpenSubtitles2018.raw.en --line-number
    441450441:- You killed my friend, my best friend, my lifelong friend!

    real    1.473
    user    1.186
    sys     0.286
    maxmem  12512 MB
    faults  0

And then after:

    $ time rg 'You killed my friend, my best friend, my lifelong friend!' OpenSubtitles2018.raw.en --line-number
    441450441:- You killed my friend, my best friend, my lifelong friend!

    real    1.532
    user    1.280
    sys     0.250
    maxmem  12512 MB
    faults  0

But perf is just about in the same ballpark. That's good enough for me
at the moment in order to drop the extra dependency.

I did this because the marginal cost of adding the Iterator::count()
specialization to memchr was extremely small.

Cargo.lock

@@ -40,12 +40,6 @@ dependencies = [
  "serde",
 ]
 
-[[package]]
-name = "bytecount"
-version = "0.6.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2c676a478f63e9fa2dd5368a42f28bba0d6c560b775f38583c8bbaa7fcd67c9c"
-
 [[package]]
 name = "cc"
 version = "1.0.83"
@@ -215,12 +209,12 @@ name = "grep-searcher"
 version = "0.1.11"
 dependencies = [
  "bstr",
- "bytecount",
  "encoding_rs",
  "encoding_rs_io",
  "grep-matcher",
  "grep-regex",
  "log",
+ "memchr",
  "memmap2",
  "regex",
 ]