README: update and add examples

darwin: implement readdir_r and closedir instead of linking to the stdlib

README: update with new Darwin perf numbers (now 2x faster)

Correctly follow symlinks, fix adapter funcs, fix Windows tests

This also tunes DefaultNumWorkers for macOS/Darwin which results in a
22% speed up:
```
name         old time/op    new time/op    delta
FastWalk-10    16.8ms ± 1%    13.0ms ± 2%  -22.20%  (p=0.008 n=5+5)

name         old alloc/op   new alloc/op   delta
FastWalk-10    2.13MB ± 0%    2.14MB ± 0%     ~     (p=0.151 n=5+5)

name         old allocs/op  new allocs/op  delta
FastWalk-10     37.7k ± 0%     37.7k ± 0%   -0.01%  (p=0.016 n=5+5)
```

README: update and add example

export StatDirEntry

The StatDirEntry can be used to get the results of os.Stat for a
DirEntry.

unix: uncomment line in test

remove unused testdata folder

unix: consolidate unix (non-macOS) specific files

linux: speed up direntNamlen

Use fs.DirEntry and fs.FileInfo instead of os.DirEntry and os.FileInfo

darwin: improve readDir test coverage

dirent: cache Stat results for portableDirent

tests: fix TestEntryFilter on Windows

Add README

EntryFilter: use multiple entry stores to reduce lock contention

```
benchmark                                      old ns/op     new ns/op     delta
BenchmarkEntryFilter_Seen-10                   30.8          33.4          +8.41%
BenchmarkEntryFilter_Seen_Parallel-10          130           60.4          -53.58%
BenchmarkEntryFilter/MostlyHits-10             41.7          48.6          +16.51%
BenchmarkEntryFilter/MostlyHitsParallel-10     132           71.2          -45.89%
BenchmarkEntryFilter/HalfMisses-10             115           135           +17.54%
```

Makefile: only run nogetdirentries test on darwin

Makefile: fix bench_comp target

rename seen_* entry_filter_*

darwin: fixup build tags on zsyscall_*getdirentries* files

remove lstatDirent as it's no longer needed since we cache the result

Add Makefile and benchmark script

benchmark fastwalk against filepath.WalkDir and godirwalk.Walk

Below results from a 2021 M1 MacBook Pro.

filepath vs. fastwalk:
```
name               old time/op    new time/op    delta
WalkComparison-10    30.8ms ± 0%    17.4ms ± 1%  -43.43%  (p=0.008 n=5+5)

name               old alloc/op   new alloc/op   delta
WalkComparison-10    4.33MB ± 0%    2.14MB ± 0%  -50.64%  (p=0.008 n=5+5)

name               old allocs/op  new allocs/op  delta
WalkComparison-10     50.9k ± 0%     37.7k ± 0%  -26.00%  (p=0.008 n=5+5)
```

godirwalk vs. fastwalk:
```
name               old time/op    new time/op    delta
WalkComparison-10    59.2ms ± 1%    17.4ms ± 1%  -70.63%  (p=0.008 n=5+5)

name               old alloc/op   new alloc/op   delta
WalkComparison-10    25.2MB ± 0%     2.1MB ± 0%  -91.54%  (p=0.008 n=5+5)

name               old allocs/op  new allocs/op  delta
WalkComparison-10     57.6k ± 0%     37.7k ± 0%  -34.58%  (p=0.016 n=4+5)
```

darwin: don't export Getdirentries function

DirEntry: cache results on unix and ensure fastwalk.DirEntry is always used

This commit changes the unix DirEntry implementation to cache the
results of Info() and Stat(). It also ensures that the fs.DirEntry
passed to WalkFunc is always a fastwalk.DirEntry, which has an extra
Stat() method.

fastwalk: skip ErrPermission test on Windows

TODO: create an applicable test for Windows

github: remove go1.16 target

github: add workflows for Linux, macOS, and Windows

add MIT LICENSE and reference Go's LICENSE

darwin: use getdirentries64 for readDir

Use getdirentries64 for readDir since it reduces the number of syscalls.
Since getdirentries64 is deprecated and banned by the App Store the
'nogetdirentries' build tag can be used to omit its usage and linkage.

convert Walk() to take a fs.WalkDirFunc

initial commit