StringByteBufferEncoder: Encode strings to a ByteBuffer (optimization)

This makes ProtoBench approximately 10% faster for both Speed and Size options. Unfortunately, this is a "risky" change, since it touches a lot of code. But in my tests it does make things go quite a bit faster.

This version no longer affects the Lite runtime.

[Evan Jones, 2010-04-28 18:32:58]

I mentioned this in the protobuf mailing list. I'm unsure if this optimization
makes sense, but it does appear to improve performance. If this seems
worthwhile, I'll clean up StringEncoder and test it more thoroughly.

[Evan Jones, 2010-06-06 15:17:13]

I've revised this patch to no longer affect the lite runtime. It might be worth
considering including this version. My tests show that this is 5-20% faster on
ProtoBench, for both Size and Speed messages. The downside is that this is a
fairly "invasive" change, and while I've attempted to test it carefully, it
still could introduce bugs.

But it is quite a bit faster.

Again: I won't be disappointed if it is decided that this is not appropriate to
be included in the main protocol buffer distribution. But I'll try to maintain
this patch anyway.