allow returning []byte in place of string fields

[glycerine]

On v1, I was seeing alot of allocation due to string copying. So I added 2 small upgrades:

a) BenchmarkUnmarshalCapnpZeroCopyNoAlloc() in mem.go

b) Bytes() and TextList.AtBytes() methods that return the []byte slice directly instead of copying it to a string.

The reduction in memory allocation means that I can then read a ton of mmap-ed data and analyze the string fields, all while doing zero-allocation and having zero GC pressure.

See glycerine/go-capnproto@db36ab2

comparisons:

BenchmarkUnmarshalCapnp-4                    2000000           605 ns/op     766.08 MB/s         256 B/op          5 allocs/op              
BenchmarkUnmarshalCapnpZeroCopy-4           10000000           182 ns/op    2539.81 MB/s          88 B/op          3 allocs/op              
BenchmarkUnmarshalCapnpZeroCopyNoAlloc-4    100000000           24.9 ns/op  18668.12 MB/s          0 B/op          0 allocs/op   

and

BenchmarkWithoutStringBytes-4               20000000            78.6 ns/op        32 B/op          1 allocs/op   
BenchmarkStringBytes-4                      50000000            29.4 ns/op         0 B/op          0 allocs/op                              

It would be great to add similar API []byte access to v2.