The 'eval' function was renamed to 'repl'.

The code to refresh the $PATH was moved into it's own function:
'prefresh', and is now being called in three locations:
  - At the beginning of 'repl', before the main loop
  - Inside the main loop, possibly during a path search if the command
    is not initially found
  - Inside the main loop, if the call to Command::spawn() throws an
    error, and the error is ErrorKind::NotFound

Doc comments were added for each function, as well as a few more
comments throughout that detail the program's control flow.

The error messages in the main repl loop were cleaned up to have a more
consistent style, and to provide more/better detail.

Adds two additional error checks when the shell forks:
1. Checks for permission (+r, +x)
2. Checks if the file exists

The first error may occur if the user does not have read access to the
file, or if the file is not executable. The second error may occur if a
file was removed from the $PATH, and the $PATH hasn't been refreshed
yet.

This is a modified implementation of the 'refresh path on every
iteration of the loop' idea. It instead, only refreshes the path if the
command is not found. After the first refresh, if the command still is
not found, it throws and error.

Implements the path refresh at the start of each REPL loop. On this
commit, it is printing out how long it needed to refresh all the paths
in milliseconds.

Allow user to input a fullpath or relative path to a file that should be
forked to.

Make the control flow in our cd implementation a bit easier to
read/follow.

Print a newline before quitting when we receive an EOF.

Instead of printing an error if the path is not specified, simply 'cd'
into the user's home directory instead.

Added logic to change directories with 'cd'.

Added logic to capture the interrupt signal. Using a rust crate called
'ctrlc' to do the heavy lifting. Program will simply reprint the prompt
on a new line if the interrup signal is detected.