internal/prefix: use wide integer loads when reading
internal/prefix: use wide integer loads when reading

Rather than reading bits into the bit-buffer on a byte-by-byte basis,
use uint64 loads to get a wide integer in a single operation.
This optimization is only done on the Reader. The equivalent change is
still to come for the Writer.

On the compress/flate benchmarks:
benchmark                                     old MB/s     new MB/s     speedup
BenchmarkDecode/Digits/Speed/1e4-4            102.85       112.07       1.09x
BenchmarkDecode/Digits/Speed/1e5-4            97.34        106.39       1.09x
BenchmarkDecode/Digits/Speed/1e6-4            100.97       110.31       1.09x
BenchmarkDecode/Digits/Default/1e4-4          100.51       108.91       1.08x
BenchmarkDecode/Digits/Default/1e5-4          97.16        105.93       1.09x
BenchmarkDecode/Digits/Default/1e6-4          97.60        105.35       1.08x
BenchmarkDecode/Digits/Compression/1e4-4      100.38       109.33       1.09x
BenchmarkDecode/Digits/Compression/1e5-4      97.86        105.72       1.08x
BenchmarkDecode/Digits/Compression/1e6-4      97.04        105.08       1.08x
BenchmarkDecode/Huffman/Speed/1e4-4           98.97        112.76       1.14x
BenchmarkDecode/Huffman/Speed/1e5-4           109.61       127.67       1.16x
BenchmarkDecode/Huffman/Speed/1e6-4           110.23       128.10       1.16x
BenchmarkDecode/Huffman/Default/1e4-4         99.20        110.47       1.11x
BenchmarkDecode/Huffman/Default/1e5-4         102.74       117.63       1.14x
BenchmarkDecode/Huffman/Default/1e6-4         104.49       120.32       1.15x
BenchmarkDecode/Huffman/Compression/1e4-4     98.83        113.05       1.14x
BenchmarkDecode/Huffman/Compression/1e5-4     102.10       117.41       1.15x
BenchmarkDecode/Huffman/Compression/1e6-4     104.28       120.21       1.15x

dsnet/compress,dsnet/compress