Optimize readVarString.

Before:

BenchmarkReadVarStr4     1000000              1698 ns/op
BenchmarkReadVarStr10    1000000              1812 ns/op

After:

BenchmarkReadVarStr4     2000000               853 ns/op
BenchmarkReadVarStr10    5000000               712 ns/op

This is part ef the ongoing effort to optimize serialization as noted in
conformal/btcd#27.
Add benchmarks for writeVarString.

This commit adds a couple of benchmarks for the writeVarString function.