writer: optimize little-endian writes by removing loop

benchmark                                  old ns/op    new ns/op    delta
BenchmarkLittleEndianWriteUint32Align1         26351        23401  -11.20%
BenchmarkLittleEndianWriteUint32Align32        18301        12165  -33.53%
BenchmarkLittleEndianWriteUint64Align1         24981        20021  -19.86%
BenchmarkLittleEndianWriteUint64Align32        22311        16080  -27.93%
BenchmarkLittleEndianWriteUint64Align64        19471        12790  -34.31%

benchmark                                   old MB/s     new MB/s  speedup
BenchmarkLittleEndianWriteUint32Align1        155.44       175.03    1.13x
BenchmarkLittleEndianWriteUint32Align32       223.81       336.68    1.50x
BenchmarkLittleEndianWriteUint64Align1        163.96       204.58    1.25x
BenchmarkLittleEndianWriteUint64Align32       183.58       254.71    1.39x
BenchmarkLittleEndianWriteUint64Align64       210.36       320.23    1.52x
writer: optimize big-endian writes a tiny bit

benchmark                               old ns/op    new ns/op    delta
BenchmarkBigEndianWriteUint32Align1         19941        19441   -2.51%
BenchmarkBigEndianWriteUint32Align32        10660        10605   -0.52%
BenchmarkBigEndianWriteUint64Align1         17501        17090   -2.35%
BenchmarkBigEndianWriteUint64Align32        14190        14130   -0.42%
BenchmarkBigEndianWriteUint64Align64        11610        11405   -1.77%

benchmark                                old MB/s     new MB/s  speedup
BenchmarkBigEndianWriteUint32Align1        205.40       210.69    1.03x
BenchmarkBigEndianWriteUint32Align32       384.22       386.21    1.01x
BenchmarkBigEndianWriteUint64Align1        234.04       239.66    1.02x
BenchmarkBigEndianWriteUint64Align32       288.64       289.86    1.00x
BenchmarkBigEndianWriteUint64Align64       352.78       359.12    1.02x