Use generated code for Vitess bson benchmarks.

The default vitess bson API uses reflection, which is slow. Vitess also
provides a code generator (bsongen), which I used to generate a custom
encoder/decoder.
I started off with this:
BenchmarkVitessBsonMarshal        100000             16843 ns/op
BenchmarkVitessBsonUnmarshal      500000              5185 ns/op

Reduced the default excessive buffer allocation from 16K to 1K:
BenchmarkVitessBsonMarshal        500000              4772 ns/op
BenchmarkVitessBsonUnmarshal      500000              5416 ns/op

Used generated code:
BenchmarkVitessBsonMarshal       1000000              2083 ns/op
BenchmarkVitessBsonUnmarshal     1000000              1158 ns/op

We'll work on reducing the default buffer size from 16K to 1K. It will
take us a few iterations to get there.

PS: Accept this CL only if you think it's fair, because other APIs don't use generated code.