Fix C++ perftest compilation
Don't construct a Vec in read_raw_varint64

Nearly halving the runtime of some perftests.
Optimize CodedOutputStream

This makes the internal buffer fixed size and doesn't initialize its
contents, saving a memset that comprised 20% of the runtime of one of the
perf tests.
Optimize CodedInputStream

The blocking read from stdin() is gone in the mean time. Create a buffer
of 4k size to greatly speed up reading. It is on par with C++ in some
cases now.