Add encoder dictionary support.

Currently uses the dictionary content and literal compression table.

The only unused elements are the FSE tables.

```
BenchmarkEncodeAllDict/length-19-level-fastest-dict-1
BenchmarkEncodeAllDict/length-19-level-fastest-dict-1-32         	  141180	      8351 ns/op	   2.28 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-19-level-default-dict-1
BenchmarkEncodeAllDict/length-19-level-default-dict-1-32         	   19639	     60085 ns/op	   0.32 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-19-level-better-dict-1
BenchmarkEncodeAllDict/length-19-level-better-dict-1-32          	    5714	    196535 ns/op	   0.10 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-12131-level-fastest-dict-1
BenchmarkEncodeAllDict/length-12131-level-fastest-dict-1-32      	   19077	     59129 ns/op	 205.16 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-12131-level-default-dict-1
BenchmarkEncodeAllDict/length-12131-level-default-dict-1-32      	    3999	    301825 ns/op	  40.19 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-12131-level-better-dict-1
BenchmarkEncodeAllDict/length-12131-level-better-dict-1-32       	    2352	    495323 ns/op	  24.49 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-210569-level-fastest-dict-1
BenchmarkEncodeAllDict/length-210569-level-fastest-dict-1-32     	    1052	   1115971 ns/op	 188.69 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-210569-level-default-dict-1
BenchmarkEncodeAllDict/length-210569-level-default-dict-1-32     	     673	   1763746 ns/op	 119.39 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-210569-level-better-dict-1
BenchmarkEncodeAllDict/length-210569-level-better-dict-1-32      	     423	   2820325 ns/op	  74.66 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-102605-level-fastest-dict-1
BenchmarkEncodeAllDict/length-102605-level-fastest-dict-1-32     	    2352	    494047 ns/op	 207.68 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-102605-level-default-dict-1
BenchmarkEncodeAllDict/length-102605-level-default-dict-1-32     	    1142	   1049912 ns/op	  97.73 MB/s	       0 B/op	       0 allocs/op
BenchmarkEncodeAllDict/length-102605-level-better-dict-1
BenchmarkEncodeAllDict/length-102605-level-better-dict-1-32      	     662	   1791540 ns/op	  57.27 MB/s	       0 B/op	       0 allocs/op
```