Using int was a mistake. This patch changes String, StringImpl,
StringView and StringBuilder to use size_t instead of int for lengths.
Obviously a lot of code needs to change as a result of this.
This was unnecessarily confusing. When we build up a chain of commands
connected by pipes, we now store the file descriptors of each end of
these pipes as rewirings in a vector. The rewirings are then put into
effect by calls to dup2().
