bug #117: speed up inflate_fast

bug #117: speed up inflate_fast

Based on a patch by Nigel Tao:
https://github.com/madler/zlib/pull/292/commits/e0ff1f330cc03ee04843f857869b4036593ab39d

This patch makes unzipping of files up to 1.2x faster on x86_64.  The other part
(1.3x speedup) of the patch by Nigel Tao is unsafe as discussed in the review of
that pull request.  zlib-ng already has a different way to optimize the memcpy
for that missing part.

The original patch was enabled only on little-endian machines. This patch adapts
the loading of 64 bits at a time to big endian machines.

Benchmarking notes from Hans Kristian Rosbach:
https://github.com/zlib-ng/zlib-ng/pull/224#issuecomment-444837182

Benchmark runs: 7, tested levels: 0-7, testfile 100M

develop at 796ad10 with -O3:
Level  Comp   Comptime min/avg/max  Decomptime min/avg/max
0     100.02%       0.01/0.01/0.02          0.08/0.09/0.11
1      47.08%       0.49/0.50/0.51          0.37/0.39/0.40
2      36.02%       1.10/1.12/1.13          0.39/0.39/0.40
3      34.77%       1.32/1.34/1.37          0.38/0.38/0.38
4      33.41%       1.50/1.53/1.56          0.37/0.37/0.38
5      33.07%       1.85/1.87/1.90          0.36/0.37/0.38
6      32.83%       2.54/2.57/2.61          0.36/0.37/0.38
avg    45.31%                 1.28                   0.34
tot                          62.60                  16.58

PR224 with -O3:
Level  Comp   Comptime min/avg/max  Decomptime min/avg/max
0     100.02%       0.01/0.01/0.02          0.09/0.09/0.10
1      47.08%       0.49/0.50/0.51          0.37/0.37/0.38
2      36.02%       1.09/1.11/1.13          0.38/0.38/0.39
3      34.77%       1.32/1.34/1.38          0.35/0.36/0.38
4      33.41%       1.49/1.52/1.54          0.36/0.36/0.37
5      33.07%       1.85/1.88/1.93          0.35/0.36/0.37
6      32.83%       2.55/2.58/2.65          0.35/0.35/0.36
avg    45.31%                 1.28                   0.33
tot                          62.48                  16.02

So I see about a 5.4% speedup on my x86_64 machine, not quite the 1.2x speedup
but a nice speedup nevertheless. This benchmark measures the total execution
time of minigzip, so that might have caused some inefficiencies.

At -O2, I only see a 2.7% speedup.