Grow repeated arrays more slowly

The previous aggressive growth was requiring 25MB allocations when more
than 4k items were present in a repeated.
Don't over allocate fields

It leads to excessive memory use.
Revert to the previous repeated field start size

Don't make small repeated fields pay an extra memory use penalty beyond
what they were paying before.
Add Cython generated .c code
Version bump
Merge branch 'master' into less-memory
Merge pull request #30 from dowski/less-memory

Less memory