Add benchmarks for writeVarInt.

This commit adds a few benchmarks for the writeVarInt function.
Optimize writeVarInt.

Before:

BenchmarkWriteVarInt1   10000000               173 ns/op
BenchmarkWriteVarInt3    2000000               965 ns/op
BenchmarkWriteVarInt5    2000000               966 ns/op
BenchmarkWriteVarInt9    2000000               968 ns/op

After:

BenchmarkWriteVarInt1   20000000               101 ns/op
BenchmarkWriteVarInt3   20000000               136 ns/op
BenchmarkWriteVarInt5   10000000               142 ns/op
BenchmarkWriteVarInt9   10000000               156 ns/op

This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27.