feat: optimise reading ints and longs

[nrwiersma]

This PR optimises the Reader.ReadInt and Reader.ReadLong functions.

The benchmarked change is as follows:

name                      old time/op    new time/op    delta
SuperheroDecode-8            265ns ± 1%     244ns ± 1%  -7.95%  (p=0.000 n=10+10)
SuperheroEncode-8            200ns ± 0%     204ns ± 2%  +2.15%  (p=0.000 n=10+10)
PartialSuperheroDecode-8     120ns ± 0%     109ns ± 1%  -9.34%  (p=0.000 n=10+10)
SuperheroGenericDecode-8    23.8µs ± 1%    23.6µs ± 0%  -0.69%  (p=0.000 n=10+8)
SuperheroGenericEncode-8     268ns ± 3%     252ns ± 0%  -6.01%  (p=0.000 n=10+10)
SuperheroWriteFlush-8        164ns ± 1%     164ns ± 0%  +0.43%  (p=0.002 n=9+10)

name                      old alloc/op   new alloc/op   delta
SuperheroDecode-8            47.0B ± 0%     47.0B ± 0%    ~     (all equal)
SuperheroEncode-8             112B ± 0%      112B ± 0%    ~     (all equal)
PartialSuperheroDecode-8     9.00B ± 0%     9.00B ± 0%    ~     (all equal)
SuperheroGenericDecode-8    8.37kB ± 0%    8.37kB ± 0%    ~     (all equal)
SuperheroGenericEncode-8     80.0B ± 0%     80.0B ± 0%    ~     (all equal)
SuperheroWriteFlush-8        0.00B          0.00B         ~     (all equal)

name                      old allocs/op  new allocs/op  delta
SuperheroDecode-8             0.00           0.00         ~     (all equal)
SuperheroEncode-8             1.00 ± 0%      1.00 ± 0%    ~     (all equal)
PartialSuperheroDecode-8      0.00           0.00         ~     (all equal)
SuperheroGenericDecode-8       303 ± 0%       303 ± 0%    ~     (all equal)
SuperheroGenericEncode-8      3.00 ± 0%      3.00 ± 0%    ~     (all equal)
SuperheroWriteFlush-8         0.00           0.00         ~     (all equal)