Speed up encoding for int64, float32/64, and bool

[erykwalder]

Before:

BenchmarkEncodeInt64     5000000           301 ns/op
BenchmarkEncodeFloat64   2000000           892 ns/op
BenchmarkEncodeBool  5000000           255 ns/op

After:

BenchmarkEncodeInt64    10000000           128 ns/op
BenchmarkEncodeFloat64   5000000           266 ns/op
BenchmarkEncodeBool 10000000           124 ns/op

Relatively minor performance wins, but I thought I'd see what others thought.

[cbandy]

Elsewhere we use strconv.AppendBool(). We should probably do one or the other, not both.

[erykwalder]

Updated benchmark:

BenchmarkEncodeBool 10000000           124 ns/op

[johto]

Looks great. I'll merge this after #351.

[johto]

Pushed in 8910d1c. Thanks!