1) Inline EncodeFixed{32,64}(). They emit single machine instructions on 64-bit processors.
2) Remove size narrowing compiler warnings from DecodeFixed{32,64}().
3) Add comments explaining the current state of optimizations in compilers we care about.
4) Change C-style includes, like <stdint.h>, to C++ style, like <cstdint>.
5) memcpy -> std::memcpy.

The optimization comments are based on https://godbolt.org/z/RdIqS1. The missed optimization opportunities in clang have been reported as https://bugs.llvm.org/show_bug.cgi?id=41761

The change does not have significant impact on benchmarks. Results below.

LevelDB:    version 1.22
Date:       Mon May  6 10:42:18 2019
CPU:        72 * Intel(R) Xeon(R) Gold 6154 CPU @ 3.00GHz
CPUCache:   25344 KB
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)

With change
------------------------------------------------
fillseq      :       2.327 micros/op;   47.5 MB/s
fillsync     :    4185.526 micros/op;    0.0 MB/s (1000 ops)
fillrandom   :       3.662 micros/op;   30.2 MB/s
overwrite    :       4.261 micros/op;   26.0 MB/s
readrandom   :       4.239 micros/op; (1000000 of 1000000 found)
readrandom   :       3.649 micros/op; (1000000 of 1000000 found)
readseq      :       0.174 micros/op;  636.7 MB/s
readreverse  :       0.271 micros/op;  408.7 MB/s
compact      :  570495.000 micros/op;
readrandom   :       2.735 micros/op; (1000000 of 1000000 found)
readseq      :       0.118 micros/op;  937.3 MB/s
readreverse  :       0.190 micros/op;  583.7 MB/s
fill100K     :     860.164 micros/op;  110.9 MB/s (1000 ops)
crc32c       :       1.131 micros/op; 3455.2 MB/s (4K per op)
snappycomp   :       3.034 micros/op; 1287.5 MB/s (output: 55.1%)
snappyuncomp :       0.544 micros/op; 7176.0 MB/s

Baseline
------------------------------------------------
fillseq      :       2.365 micros/op;   46.8 MB/s
fillsync     :    4240.165 micros/op;    0.0 MB/s (1000 ops)
fillrandom   :       3.244 micros/op;   34.1 MB/s
overwrite    :       4.153 micros/op;   26.6 MB/s
readrandom   :       4.698 micros/op; (1000000 of 1000000 found)
readrandom   :       4.065 micros/op; (1000000 of 1000000 found)
readseq      :       0.192 micros/op;  576.3 MB/s
readreverse  :       0.286 micros/op;  386.7 MB/s
compact      :  635979.000 micros/op;
readrandom   :       3.264 micros/op; (1000000 of 1000000 found)
readseq      :       0.169 micros/op;  652.8 MB/s
readreverse  :       0.213 micros/op;  519.5 MB/s
fill100K     :    1055.367 micros/op;   90.4 MB/s (1000 ops)
crc32c       :       1.353 micros/op; 2887.3 MB/s (4K per op)
snappycomp   :       3.036 micros/op; 1286.7 MB/s (output: 55.1%)
snappyuncomp :       0.540 micros/op; 7238.6 MB/s
PiperOrigin-RevId: 246856811