Updated the docs for repl() in main.rs, to include the 'at_prompt'
function parameter.

Previously, if a directory could not be read, dwvsh would panic.
Instead of panicking, dwvsh should continue processing the other
directories on the $PATH.

Rename the path::prefresh() function to path::refresh(). Calling
convention should be to `use crate::path;` or `mod path;`, and then call
path::refresh(...), for verbosity.

Don't need double quotes if just printing a newline.

Add back tests for Rune::Read, Rune::Write, and Rune::Addendum,
following the new parser output. Also, renames some tests, since Read,
Write, and Addendum are no longer meters.

Remove some commented out code, that is no longer needed after the
parser overhaul.

Add docstring for `incant_quiet_io`.

Remove println!() in main that was used for debugging the parser output.

Fix the ~ test, so that it uses env!("HOME"), instead of my hard-coded
home directory.

Rebuilt the LR parser (i.e. read()) from the ground up. This required
that some changes be made to recite(), in order to accomodate the new
data structures. These data structures were each split out into their
own file, in order to make working with each component a bit easier.

In addition to reworking the parts of the parser already present, some
new features were also added, such as:
  - Support for strings (' and ")
  - Support for environment variables ($)
  - Support for interpreting tild as $HOME (~)
  - Support for sub-reading and sub-reciting (`)