Improve performance

[koute]

This isn't very high priority as serialization/deserialization isn't really a bottleneck in Substrate AFAIK, although the current implementation leaves some performance on the table.

For reference, I've hooked up parity-scale-codec to the rust_serialization_benchmark benchmarks and here are the results compared to speedy which is my personal serialialization/deserialization crate that I've optimized for speed and with almost exactly the same serialization format as parity-scale-codec (it doesn't special-case Option<bool> and has a different varint format, but the rest should serialize the same, sans defaults). Here are the results on my machine:

benchmark	parity-scale-codec	speedy
log/serialize	552.15us	157.79us
log/deserialize	3.05ms	1.74ms
mesh/serialize	3.69ms	400.30us
mesh/deserialize	10.45ms	489.41us
minecraft_savedata/serialize	534.54us	224.66us
minecraft_savedata/deserialize	2.56ms	1.40ms