Update the package to use generics (#16)

Notes:

- The Entry interface now has a type parameter, so that it can talk
  about its own underlying type.
- Methods which relied on being able to return nil for the entry now
  return a pointer to the type.
- We now use the any alias instead of interface{} anywhere it was left
  after adding type parameters.
- In tests, instances of `assert.Equal(t, nil, ...)` have been changed
  to `assert.Nil(t, ...)`. This is because assert.Equal does not
  consider the untyped nil constant to be equal to a nil pointer of a
  particular type; probably it worked before because the other argument
  was an interface.

Benchmarks are broadly similar. Before:

goos: linux
goarch: amd64
pkg: github.com/raviqqe/hamt
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
BenchmarkHamtInsert-8               	     195	   6086113 ns/op
BenchmarkHamtDelete-8               	     710	   1626577 ns/op
BenchmarkHamtFirstRestIteration-8   	       1	4049853564 ns/op
BenchmarkHamtForEachIteration-8     	    9024	    112623 ns/op
BenchmarkBuiltinMapForEach-8        	   13258	     87643 ns/op
BenchmarkBuiltinSliceForEach-8      	  496905	      2403 ns/op
BenchmarkSetInsert-8                	     121	  10263234 ns/op
BenchmarkSetSize-8                  	      32	  35424252 ns/op
--- BENCH: BenchmarkSetSize-8
    set_test.go:160: 1
    set_test.go:160: 2
    set_test.go:160: 3
    set_test.go:160: 4
    set_test.go:160: 5
    set_test.go:160: 6
    set_test.go:160: 7
    set_test.go:160: 8
    set_test.go:160: 9
    set_test.go:160: 10
	... [output truncated]
PASS
ok  	github.com/raviqqe/hamt	27.370s

...And after:

goos: linux
goarch: amd64
pkg: github.com/raviqqe/hamt
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
BenchmarkHamtInsert-8               	     193	   6351027 ns/op
BenchmarkHamtDelete-8               	     733	   1539728 ns/op
BenchmarkHamtFirstRestIteration-8   	       1	4908351503 ns/op
BenchmarkHamtForEachIteration-8     	   15807	     71250 ns/op
BenchmarkBuiltinMapForEach-8        	   13308	     89030 ns/op
BenchmarkBuiltinSliceForEach-8      	  485073	      2444 ns/op
BenchmarkSetInsert-8                	     138	   9306362 ns/op
BenchmarkSetSize-8                  	      37	  30683603 ns/op
--- BENCH: BenchmarkSetSize-8
    set_test.go:160: 1
    set_test.go:160: 2
    set_test.go:160: 3
    set_test.go:160: 4
    set_test.go:160: 5
    set_test.go:160: 6
    set_test.go:160: 7
    set_test.go:160: 8
    set_test.go:160: 9
    set_test.go:160: 10
	... [output truncated]
PASS
ok  	github.com/raviqqe/hamt	30.325s

Co-authored-by: Yota Toyama <raviqqe@gmail.com>