Implement mutiple redirection.

Created by: chrisvittal

Add multiple redirections, that is, support for things like:

cmd <file1 <file2 ^>> err-log > output | cmd2

In current ion this doesn't work.

This change at current would be breaking as the statement:

tr 'x' 'y' | tr 'y' 'z' < file > file2

redirects file to the first tr command. With this change, we would redirect it to the second.

Overview

We treat a multiple redirection as it's own job. The only difference between these jobs and normal jobs is that they only read from their source(s) and then write to their sink(s).

Parsing

Alter Pipline contain a vector of PipeItems. A PipeItem consists of a Job, a Vec<Redirection> and a Vec<Input>. The above statement would be parsed into two items, cmd together with [file1, file2] as input and [err-log, output] as outputs, and cmd2 with no Inputs or Redirections

Command Generation and Redirection Setup

Added two two new variants of RefinedJob, Cat and Tee, Cat for concatenating multiple inputs, Tee for splitting multiple outputs. Alter the generate_commands function to return a Vec<(RefinedJob, JobKind, Vec<Redirection>, Vec<Input>)>, then condense redirect_input and redirect_output into one function that takes ownership and returns a new Vec<(RefinedJob, JobKind)>. It is here that we create the Tee and Cat jobs that we need and put them in the right spot in the pipeline.

Execution

Add exec_multi_in and exec_multi_out to handle the multiple redirections. These are fairly straightforward, they just read their sources and write to their sinks. I didn't add any extra paths to exec_job as the new RefinedJob kinds are always part of a pipeline.